Thanks to Dominique Dutoit there is now a version of the
ExternalWebBrowser package with Mac support on SqueakMap or SqueakSource.

http://map1.squeakfoundation.org/sm/account/package/a2cf0479-a33d-456f-b3ed-8f312c6c439d/autoversion/4
http://kilana.unibe.ch:8888/ExternalWebBrowser/ExternalWebBrowser-dd.5.1.mcz

 From the version description:

Adds support for almost all Mac OS browsers. Requires the Applescript
package. If you have installed the ExternalWebBrowser package before 
installing
the Applescript package, you will have to execute this command:
    ExternalWebBrowser initialize

 From the package description:

This small package should work as an alternative to scamper. It just
hands over every url to a platform specific external browser.
Currently works on:
    * Windows (IE, Firefox)
    * Linux/Unix (Konqueror, Mozilla)
    * Mac (Requires Applescript package)

Alex