...if someone "extracts" something back out of Squeak he [should] be bound by SqueakL. Otherwise, I guess there would be... a mess.
If we want that, it seems to me we can only accept exclusively SqueakL-licensed code.
Not necessarily. There is SM after all. If a package is at SM under some license you can get it from there under the license you want. This would simplify the in-image situation slightly. E.g., a simple bottom line might be that whatever gets shipped with the Squeak image comes under Squeak-L. If you don't like that, go to SM and see if there's another license, and if so, download and install on your own.
For dual-licensed packages this would be trivial. For those which aren't we can still sublicense the package under Squeak-L if the original license allows it.
Cheers, - Andreas