Hi Burr, yup those are things I want to work on. Extend surface is available in the current geometry library it's just something I haven't had a chance to get to yet. It is actually possible to use the Flow command to accomplish it right now though, see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=9732.4

Basically you make 2 planes, one for the base surface and one for the object to deform and you make the one to deform larger so it spills over outside the base surface. Then when you do the flow it will similarly spill over outside the target surface.

Merge surfaces is waiting for a larger plan on overhauling Join. I want to try and include joining and merging operations in one single command if that's possible. I have some ideas but it will need some experimentation.

re: Extend surfaces in Rhino - there is an ExtendSrf command in Rhino for that.

re: MatchSrf - that's something that is not handled by MoI's current geometry library and is pretty difficult. I would like to have it in MoI too but that one will probably take longer.

- Michael