Yes! I'm very interested in this as well!
One piece of information to add: it might be worth investigating something
more general like the configuration files used by ELLCC in controlling their cross-compilers.
This was somewhat recently discussed here:
And looks like the most recent description of it is here:
This would be rather useful in Nix for creating cross-compilers as well.
AFAIK this isn't immediately suited to your needs but I think that it's the right direction to go
and it'd be ideal to support both use-cases if we are talking about upstream support.
~Will