It should work on most KT333 boards. There may be a few that it doesn't, but they'd have to have some serious problems. Changing the 5th FID should give you the higher range of multipliers (at the cost of the lower ones). I don't think the ulti thing is a neceaasarily a chipset limitation, it is mare of a motherboards problem. Look ar all the Nforce 2s that don't correctly set FID[4]... And my KT400 board does it just fne.
edit: If you do it correctly and don't short something out, the worst that will happen if the board doesn't work is that it either won't post until you undo your mod, or, more likely, it will just stay at the <= 12.5 multis.