Well these take 6mm belts (which I snapped a little easily) but still stand 16mm tall.

http://www.technobotsonline.com/pulleys ... 5x6mm.html

I guess I need a pulley with no boss. Not dealing with HPC again, any other suppliers out there?

The alternative would be to get a low profile brushless. My only reservations would be I've just bought one that isn't, and I struggled to find one around the 300w mark that was low profile.