Why not use an actual mortising chisel such as:

http://www.hartvilletool.com/product/11707

They look pretty nice. I first choked at the price, but then thought
that 1-2 metric sizes would probably work. I never seem to end up
with sizes like 0.500 or 1.250 anyway. Looks like something for
birthday unless... I "need" one for the next project!

Thanks for the input.....