big10p - I thought you could not pick or collide with a sprite using poly detection but had to use sphere or box detection. that's what the manual says and what, indeed, i have found in practice. Have you been able to do it?
Ah, well if that's what the manual says, it must be the case. Sorry, I rarely use sprites myself and don't think I've ever tried to make one pickable. I just assumed it'd work.
I think your best bet is to use a textured quad instead (as has already been suggested), and make it poly pickable. If you need the quad to always face the camera, you can PointEntity it at the camera every update. If you do this, remember to construct the quad's triangles so they are facing the right way, otherwise PointEntity will cause the quad to always be facing away from the camera. :)