as you are well aware it'd be insanely useful if dpkg-checkbuilddeps
could tell us what dependencies need to be solved to cross compile. So I
filed bug #384694. I think doing this is pretty easy. unfortunately I
don't know perl.
Just check on -dev files. those depend on the libraries anyways so
shouldn't be build-dependencies.
Next check if the package is arch:all. if it is let it be handled the
way it always was.
if it is for host_arch depend on the -arch-cross package instead. if the
package depends on libc6-dev, for example depend on libc6-dev-arm-cross
The dpkg-checkbuilddeps file is a short shell script that is easy to
read and understand. So if you know any perl please give it a try.