So, I have an idea how we could cope with that all (and fix one more problem). My idea:

We can use files in debian/autobuilder

Files there:

sbox - legal values: {hathor, apophis}. Which sbox should be used, as thumb might sometime need apophis (embedlite-components) and sometimes hathor (Qt apps)

devkits - e.g. svn:git:debian-squeeze - it would fix the problem when shlibs:Depends fail to be created with the squeeze devkit. Plus you might disable devkits which complicate stuff. Passed to sb-conf when creating the autobuilder target.

thumb - legal values {thumb, non-thumb, both} - if an app needs newer gcc, or fails with thumb

... Feel free to add more options ...

For now we could enable only for free packages, as it would require absolutely no filtering - everything is built right there right now. And about non-free. A dirty workaround would be to check the libgcc1 version (or libstdc++6)

And what's more: it would finally allow to push CSSU dependant packages to extras-* !

/edit: The thumb option makes sense if the autobuilder automatically added the +thumb suffix. Otherwise, one'd need to enter it manually.