The current design is the Binary packages can contain an additional control field: Built-Using.
First of all, thanks very much for taking care of it, that probably will get us going.
I just would like to point out that current design solves half of the problem (being GPL compliant),
No, it solves all of the problem, cos ....
but it does not solve code duplication in the archive,
this is a different thing entirely. B-U is solely mean of ensuring to do what the licenses want us to do. Getting sanity into packages providing source around and duplication is different to that.