First, I'm really impressed with many aspects of this program, particularly the way it simulates paint volume on the canvas.
So here's my solution for the need for straight lines and misc. masking issues: masking tape.
Take a look at the attatched screen grab for a rough example, but in essence, the user could apply the tape just like one would on real canvas, then paint away, and when finished, the tape could be pulled off. I think you'd need two basic parameters to begin with, the first is width of course, and the second would be a bend resistance. In otherwords, with a bend resistance of say 100% the tape would be applied in straight lines, while at lesser percenteges it would be more pliable so curved masking would be possible.
This would be awsome for people like me who do lots of geometric type painting.