When you are adding windows support drop by the msys2 irc channel on OFTC. We have support for qt5 using mingw with premade packages available in pacman.
A lot of effort went into qt5 support by alexey.
They do provide a mingw version to support the community of devs and are great for accepting code upstream.
With most projects however mingw targets are often somewhat neglected and broken even with release versions in some subtle ways.
Our Mingw packages are actually used more broadly by devs on a daily basis and tested regularly by a package manager with other projects that depend on qt5.
Fixes end up in msys2 before they reach qt5 upstream because of that, some times we have too many as you can see here.
https://github.com/Alexpux/MINGW-packages/tree/master/mingw-...
We also have static builds and 64bit support for the mingw-w64 target as previously mentioned by jsheard