I think minib3d would be the perfect language to create a 'Maplet II' program with. CSG anyone?
fredborg is working on a new version of GILE[s] that is supposed to be multi-platform. That will take care of the lightmapping part.
I am not aware of any level-design. level editor type programs that export to .b3d. In fact, I am not aware of any program that is Mac OSX that will export a .b3d.
I use Unwrap3D to import/export (Windows only)
you could try these modellers though.
Wings3D
http://www.wings3d.com/Personally. I think Blender is well worth investing some time into.
http://www.blender.org/download/get-blender/