Took me a couple of days to find my way through the procedure, but I think I finally made it!
I generated the bit file for the 5i22 with ISE 13.1 and Mesa's sources using the fivei22 project file, but had to change the target device from xc3s1500-4fg320 to xc3s1000-4fg320 since I didn't find a project for the 1000.
Seems to work fine though I didn't have the chance to check the extra encoders and pwms.