Let's start the bug since I need it for blockers :).
Just a thought - is this only for completeness or because there actually is qt5 software out there that needs multilib qt5? What I need multilib for is wine and a couple of binary blobs that still haven't been ported to amd64. I don't know about any closed source software that uses qt5.
I don't understand the blocks of this bug. sni-qt doesn't need a multilib qt5 (#516488)
and what packages needs multilib versions of qt5, so that avahi and libdbusmenu-qt has to be 32bit qt5? Is qt5 multilib really needed or is it just for completeness?
There is Shashlik from KDE Team, that use core, gui and widgets 32 bit libraries.
I've implemented qt5-multilib build (using multilib_copy_sources to skip private dirs issue). Currently you can find it here (along with building qt5 examples):
Only exceptions currently are qtspeech and qtsql packages. qtspeech is non-multilib at the moment and qtsql should fail to build if it's built for non-native ABI with USE-flags "freetds" or "postgres" enabled.