In regards to app-emulation/virtualbox-guest-additions-4.2.0 I've installed this package on my 64 bit, Hardened, nomultilib setup and it works just fine. I haven't tested enabling the X USE flag as I'm running console only, but it shouldn't be masked if X pulls in something that's 32 bit, which it doesn't look like it does.
I have been using X for a while now with app-emulation/virtualbox-guest-additions, and have not had any issues.
*** This bug has been marked as a duplicate of bug 424797 ***