I've found a temporary workaround. Can anyone advise me if this is 'dangerous' for my raw files?
1. Copy .arw files from camera to computer
2. Run exiftool -lensmodel= *.arw
3. (lots of .arw_original backups are created)
4. Run alphalensinfo *.arw
5. Import into lightroom 4.
Step 2 blanks the lensmodel tag in the .arw file. Then, alphalensinfo creates the aux:lens tag (with the correct lens name in ). The LR4 import process discovers the lensmodel tag is blank and moves onto the aux:lens tag and everything appears normal in the library module, just like in LR3.
The catch is, I'm keeping duplicate backup copies of my original .arw files, just in case my little process is damaging them in some way.
Is anybody from adobe interested in this?