I'm writing these lines from a Vista X64 machine running LANDesk AV. I know from daily experience (well, not quite daily, I don't get exposed to live viruses that frequently) that real-time scanning actually works. Real-time scanning necessarily takes kernel mode code. And if, as you point out, 32 bit kernel mode code simply doesn't run on a 64 bit OS, then the conclusion must be that the real-time scanning is being done by 64 bit code.
If there is a limitation somewhere due to 32 bit code running under wow64, the kind of test that should reveal it would be something like this: turn off real-time scanning, copy the eicar test virus to the system32 folder, do an on-demand scan of system32. If the product suffers from wow64 limitations, it will not find the virus because 32 bit apps can't see the real system32 folder, they get diverted to syswow64. You need to turn off real time scanning for the test since otherwise, it will prevent you from copying the test virus.
That test is negative, i.e. the product does not suffer from this limitation. The exercise is not academic because several AV products have indeed suffered from this limitation.