John,
what I was thinking of doing was making a new body to slide the spindle into, a tight sliding fit and using a small thumbscrew to lock the spindle in place. That way, you could zero the mic then slide it down until it touches the valve and tighten the screw, locking it in place. that way it is direct acting and direct reading so there is almost no chance of error.
I am trying to think of a design for doing DOHC 4 valve heads, its a shame all valves arent the same distance apart! I suppose 2 mics mounted in a sleeves that can slide together or apart in a slot would do the trick. or maybe dummy camshafts with holes bored over the top of the valves for a mic to slide into.
I think I just had a good idea...