Until the ACR 6.7 is released, I think if you guys convert your CR2 file to DNG when you import then into LR4 it should work. You can copy your original CR2 file somewhere else for future reference if required.
I have not tried this not having the 5DmkIII but it should work.
This does indeed work, have just done it. It does add an extra step to the workflow though so hopefully the upgrde won't be far off