I do not think that a mesh can be a wallhole or wallniche because
the manual only mentions slabs and roofs. But the documentation
may be out of date because Karl was able to use a complex profile
wall to be a wallniche. Please see this thread:
http://archicad-talk.graphisoft.com/viewtopic.php?t=25226
You can use a mesh as a fill body set to fit into a regular A and B
size opening. If you are in AC 12, can't you make splay reveal openings?
I had understood that this is possible in AC 12.
Peter Devlin