"Worse, I'm already kind of dissatisfied with the iMate - it doesn't distinguish left and right for Alt/Shift/etc."
This may be a limitation of the keyboard itself. I remember looking into low level keyboard remapping over a decade ago in OS X (via ~/Library/Keyboards), and I have a vague memory of reading what was at the time an ancient piece of Apple reference material that some keyboards do not distinguish between left and right modifier keys.
On ADB Standard keyboards(M0116) left and right corresponding modifiers are logically indentical and can not be discriminated one another by the converter. This is ADB keyboard limitation, not converter's. Apple Extended keyboard(M0115/M3501) can discriminate them except for 'Command' key.
I do remember using my Extended Keyboard II on ADB Macs, and the keyboard viewer did distinguish between left and right modifier keys. This might not apply to other ADB keyboards, though.
This may be a limitation of the keyboard itself. I remember looking into low level keyboard remapping over a decade ago in OS X (via ~/Library/Keyboards), and I have a vague memory of reading what was at the time an ancient piece of Apple reference material that some keyboards do not distinguish between left and right modifier keys.