To my knowledge C2 uses a polygon collision system, rather than calculating per-pixel collisions, meaning you would need a polygon collision object, or several, that match the contents of the canvas.
It's theoretically possible to create invisible collision objects via events that overlay the drawn-on parts of the canvas, but it's not likely to be very practical outside of some special cases.
One possible approach might be to use the tile map object, set the tiles to 1x1 pixels, and create a "solid" tile with a square collision polygon. if you can paint 1x1 collision tiles over the same area your canvas brush is painting color, then it might work.
Granted I've never used the tile map for anything like that so I don't know if there would be performance issues. I expect there would be. Though I think the tile map would still be much better than a ton of 1x1 sprites, which might not even run at all.Update:
Okay I just tried the tile map setup I described, and to my astonishment, it actually runs in realtime without issue. Granted that was with 1 px wide lines. When I blocked in 50% of the 832, 480 screen with a solid mass of 1x1 collision tiles (over 200,000 collidable tiles) the game still ran smoothly. It started to slow down a bit when I got to 75% coverage (over 300,000 tiles). Practically speaking, if you could cull out the center tiles, and only make the edges of objects collidable, then you could probably make a pretty decent per pixel collision system.
I tried both 8-Direction and Platform behavior with it and it seems to work as you'd expect.
Also, the 1x1 pixel tiles are only important if you're doing something like simulating a per-pixel collidable terrain for a platformer. If super precise collision detection isn't essential, such as testing for loose overlaps with the mouse pointer, you could make the tiles 4x4 pixels and the whole setup would have 16 times fewer tiles in filled areas.(By "16 times fewer" I mean "1/16th the amount of tiles", but both ways of phrasing it sound weird, now that I'm looking at them.)
In any case, I hope that helps somewhat.