Little reminder about things:
[..]* Polygonal collisions are being worked on too and will appear in a future release.
You could always create collision-primitive shape for each sprite-object and use that. Because this is no 3D engine, the coll.prim "object" would be composed of a herd of few small separate circle-sprites, moved together by code, loosely covering the 'player' sprite's area. Use them for "is overlapping another object", which will be easier, when families become available. "Collision" family of objects.
I see no pixel level access in C2, no alpha access to create a coll. mask so far even via plugin. But did a little search to find some plugin material:
Does this "pixel perfect collision" example in Java makes sense to anyone?