Well, I've actually spent 3 days trying to get Qt5 to cross-compile for a custom arm platform. It might be easier for something common like the raspberry pi, but still, you may have dodged a bullet there. ;)

The worst thing about cross-compiling Qt is that I always run into different issues with new releases, and it's not even always consistent between builds. I ended up renting a 64 core VPS to build Qt so that I could iterate faster though build failures, but it's still at least half a day's work to get it to build properly.