"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.