Theo - I dont think it's just the taper - Although I have been told that that is important to get right - It's also the relative strength of the metals in contact with each other. A steel bolt even of the correct taper will "ream" into the softer alloy of the wheel - At least that was my experience and I have heard from someone who kept tightening theirs until one day the wheel fell off the car ...but the bolts and centre section were still attached to the hub!!
I know that Brensports has successfully used alloy bolts from 132 Bellini wheels and Mark W also seems to have appropriate bolts from somewhere. Another topic for the pub at the weekend!!