In order to compile freecad with Qt5, we need a pyside-9999 which support Qt5 but the current development branch of pyside doesn't support Qt4 anymore.
I think the best solution is to split pyside in two slot :
one slot for the Qt4 pyside-1.2 stable
one slot for the Qt5 ( currently in dev branch)
This was implemented more than 2 months ago in the qt overlay. Pyside and shiboken live ebuilds build from the upstream 5.6 branch and have SLOT=2.
There hasn't been any official release yet, and the last time we tried the dev branch it was rather unstable, so those ebuilds are staying in the overlay for now.