Hi Frank, I know you're interested in being able to override the
repositories.  This is a simple solution for that which meets at least
three use-cases.

Example 1
Installer
     preemptRepositoryFor: 'htmlcssparser'
     with: (MCDirectoryRepository directory: (FileDirectory default /
'my-projects' / 'mc'))

Example 2
"I'm on the train with slow Wi-Fi, look in my package-cache first for
anything before going to remote repositories."
#(squeak ss ss3) do:
     [ : each | Installer
          preemptRepositoryFor: each
          with: MCCacheRepository default ]

Example 3
"I want to make a new system.  Use the default repository, ss3, but if
its down fall back to the original SqueakSource."
Installer
     fallbackRepositoryFor: #ss3
     is: #ss

On Fri, Aug 9, 2013 at 5:36 PM,  <commits at source.squeak.org> wrote:
> A new version of Installer-Core was added to project The Inbox:
> http://source.squeak.org/inbox/Installer-Core-cmm.378.mcz
>
> ==================== Summary ====================
>
> Name: Installer-Core-cmm.378
> Author: cmm
> Time: 9 August 2013, 5:04:23.858 pm
> UUID: 79285e19-2913-476e-b396-b1a01a652edc
> Ancestors: Installer-Core-cmm.377
>
> Allow the default repositories for external-packages to be preempted or reinforced.
>
> =============== Diff against Installer-Core-cmm.377 ===============
>
> Item was changed:
>   Object subclass: #Installer
>         instanceVariableNames: 'answers packages messagesToSuppress useFileIn noiseLevel currentRepository'
> +       classVariableNames: 'InstallerBindings IsSetToTrapErrors Remembered Repositories SkipLoadingTests ValidationBlock'
> -       classVariableNames: 'InstallerBindings IsSetToTrapErrors Remembered SkipLoadingTests ValidationBlock'
>         poolDictionaries: ''
>         category: 'Installer-Core'!
>
>   !Installer commentStamp: 'kph 3/30/2009 01:29' prior: 0!
>   Documentation now available at http://installer.pbwiki.com/Installer
>
>   useFileIn - flag to load source.st rather than using Monticello!
>
> Item was added:
> + ----- Method: Installer class>>defaultRepositoryFor: (in category 'repositories') -----
> + defaultRepositoryFor: anAssociation
> +       ^ (self perform: anAssociation key)
> +                project: anAssociation value ;
> +                mc!
>
> Item was added:
> + ----- Method: Installer class>>fallbackRepositoryFor:is: (in category 'repositories') -----
> + fallbackRepositoryFor: scope is: anMCRepository
> +       self
> +               overrideRepositoryFor: scope
> +               with:
> +                       (MCRepositoryGroup new
> +                                addRepository: (self defaultRepositoryFor: scope) ;
> +                                addRepository: anMCRepository ;
> +                                yourself)!
>
> Item was added:
> + ----- Method: Installer class>>overrideRepository:with: (in category 'repositories') -----
> + overrideRepository: scope with: anMCRepository
> +       "Override the standard repository specified by scope with anMCRepository.  To override an entire repository, specify scope as the repository selector.  To override a single project within a repository, scope should be an Association whose key is the repository selector and value is the project name."
> +       self repositories
> +               at: scope
> +               put: anMCRepository!
>
> Item was added:
> + ----- Method: Installer class>>packageCache (in category 'repositories') -----
> + packageCache
> +       ^ MCCacheRepository default!
>
> Item was added:
> + ----- Method: Installer class>>preemptRepository:with: (in category 'repositories') -----
> + preemptRepository: scope with: anMCRepository
> +       self
> +               overrideRepositoryFor: scope
> +               with:
> +                       (MCRepositoryGroup new
> +                                addRepository: anMCRepository ;
> +                                addRepository: (self defaultRepositoryFor: scope) ;
> +                                yourself)!
>
> Item was added:
> + ----- Method: Installer class>>removeOverride: (in category 'repositories') -----
> + removeOverride: scope
> +       "Remove override specified by scope and return to using the default repository for packages within that scope."
> +       ^ self repositories
> +               removeKey: scope
> +               ifAbsent: [  ]!
>
> Item was changed:
> + ----- Method: Installer class>>repositories (in category 'repositories') -----
> - ----- Method: Installer class>>repositories (in category 'instanciation') -----
>   repositories
> +       ^ Repositories ifNil: [ Repositories := Dictionary new ]!
> -
> -       ^ self class organization listAtCategoryNamed: 'repositories'.
> - !
>
> Item was added:
> + ----- Method: Installer class>>repositoryFor: (in category 'repositories') -----
> + repositoryFor: anAssociation
> +       | rep |
> +       rep := self repositories
> +               at: anAssociation
> +               ifAbsent:
> +                       [ self repositories
> +                               at: anAssociation value
> +                               ifAbsent:
> +                                       [ self repositories
> +                                               at: anAssociation key
> +                                               ifAbsent: [ ^ self defaultRepositoryFor: anAssociation ] ] ].
> +       ^ rep isSymbol
> +               ifTrue: [ self defaultRepositoryFor: rep -> anAssociation value ]
> +               ifFalse: [ rep ]!
>
> Item was changed:
>   ----- Method: Installer>>htmlValidator (in category 'external-packages') -----
>   htmlValidator
>         "Validates HTML and CSS pages against W3C DTD."
> +       ^ { #ss3 -> 'htmlcssparser'.
> +       'HTML' }!
> -       ^ { #ss -> 'htmlcssparser'.
> -       'HTML-tb.37' }!
>
> Item was changed:
>   ----- Method: Installer>>setRepository: (in category 'private') -----
>   setRepository: anAssociation
> +       currentRepository := self class repositoryFor: anAssociation!
> -       currentRepository := (self class perform: anAssociation key)
> -                project: anAssociation value ;
> -                mc!
>
>