I will be watching this thread with interest. I also have a Mark I that I have thought seriously about mounting a red dot to. Would rather not have the gun drilled.
Have also thought about the same thing for my Smith model 10. The no drill mounts for it are pricey, and obscene looking.
I was talking to the guy at the LGS and he told me I could probably get by nicely with a lower cost red dot for the Mark. Between being a .22, and an internal slide, the scope would not be subject to a lot of jarring on recoil.
The biggest down side to doing this for my Mark is the non availability of .22 ammo. I reload 9 and .38, but am at the mercy of the market for .22 right now.