I forgot that there are two kinds of plugins: internal and external. And 
IA32ABI is both internal and included in each Cog virtual machine.
If I download Squeak 5.0 onto my Ubuntu 15.04 laptop from the homepage 
and execute:

Smalltalk listBuiltinModules do: [:ea| Transcript cr; show: ea]

I can see IA32ABI is clearly loaded. Then I can go to 
squeaksource.com/Alien and load Alien into then execute in a Workspace.

Alien exampleCqsort.

This will produce an array of numbers.

(Time millisecondsToRun: [100 timesRepeat: [Alien exampleCqsort]]) / 100.0

This will return ~1.0.

So, it looks like trying it out is not so hard. I'm not sure what it all 
means, but I have a successful control experiment at this point to work 
with. Perhaps I need to read the documentation now to explain what I'm 
seeing?

A plugin repository would not account for internal plugins, but I still 
think it's a good idea. It could focus and foster external plugin hacking.

The issue about what kind of repository (ftp.squeak.org, GitHub, 
TravisCI) is where a URL will point. Wherever client that socket is 
accessed by, either  SqueakMap or a browser, seems to be a subordinate 
issue people could decide for themselves.

Chris