Since version 9.50, Opera is now purely 64-bit and does not require multilib; I'm also successfully using it since beta and just found it masked byt profile, but it works.
Please give a a real problem. With USE=ia32, you should still get emul-x86 libraries, whereas with USE=-ia32 you should not.
Well, for me it won't merge in both ways, as it's masked in profile.mask, which should not be needed.