Thanks for all your replies.

Lee440, you are right, it is quite astonishing that there are no people shouting HERESY (yet).

I think that the Docter sight would probably be the most suitable as it weighs less than most bullets for these rifles and is as far as I know the lowest of all.
The only question is what the ideal mount would be.
It probably should have these features:
1. it should be kind of "keyhole surgery" so the rifle is not altered too much
2. it should be VERY low, just clearing the iron sights, so that it doesn't "feel" different when you mount
3. and this would be the hardest combining it with 1&2, it should be quickly removable without changing POI when put on again.

Any ideas?

Larcher,
could you post some photos of your Chapuis?

Stoerte