In reply to:

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It just means that you must offer the full sources
from the same location from which you offer your modified
(cut-down) version. In a different tarball. So although
it'll still take up space on your server, you at least
wouldn't be forcing anyone to do a huge download.

At least this is how I understand it. I'm not a GPL expert by any stretch of the imagination.

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Right on. Take the binaries, libraries etc. that is needed
to use the package and pack that into the binary
tar-ball. This is what most users will want, unless they
don't trust you and want to inspect/compile it themselves,
or if they want to modify it further.

To comply with the GPL you also need to make the source code
available - including your changes. Pointing to the original
.deb package isn't enough as your changes aren't included
there. So essentially, tar up an archive of your project
directory and stick that on the server along with the binary
tar-ball. Stuff from the original .deb that you don't use does not need to be included.

You can not distribute it under any lesser free license than
the GPL. Not even the BSD (essentially: do what ever you
want as long as you credit the earlier developers) license,
as it (BSD license) allows the next developer to distribute without source.

The point of the GPL is to make sure the source stays available - always.

Chicken clause: At least that's the way I understand it...

/Michael