Compiling Qt with the right switches is hard enough, before you throw in cross compiling on top. Since you only need to compile a new Qt release once every few months, why optimize the process? Just kick it off and go to sleep. I don't modify core Qt libraries ever. If I was iterating on Qt itself, I would definitely figure out cross compiling. Anyways if you come up with such a working cross-compile recipe, you should post it some place.
I think people working on cross compilation are wasting their time. They are trying to fix it "properly", separating build time dependency and run time dependency, because host system can't run cross compiled target binary. The obvious solution (what Scratchbox2 does) is for host system to run cross compiled target binary, instead of trying to "fix" the build system.