The problem with bsp is that there are a couple of different bsp formats out there and it is difficult to select one. Most people including myself use their own custom BSP formats.
However, if you visit this site you will see that Cartography Shop has recently included a compiler which takes a .map file and saves it as either a Blitz3D file or the new ".DBO" from DarkBasicPro.
The reason I'm insiting with this suggestion is because I believe 3DC has a lot of potential and this would be a very appealing feature for version 6 ... :-/
A .map exporter for Canvas could *theoretically* be done. But it wouldn't be anywhere near idiotproof, and anyone who writes one will have an overflowing inbox filled with hate mail from frustrated users who don't quite grasp the difference between planes and polygons.
BSP, however, is incompatible with the kind of geometry that a 3D app like Canvas outputs. Without getting technical, it just isn't possible to make a consistent and robust BSP solution for Canvas without forcing users to learn a completely different modeling approach.