I feel like sliding a cam in the crack at the base of the roof would cause a lot of rope drag, and using a longer sling would put you right back at the bolt. Any advice before the snow melts?

I think you can sling a knob on the lip of the roof from below. Maybe hidden in this pic by the climber, or the one the foot is on.

Climbed it yesterday - caughtinside is right - i slung the knob from below - gives you that extra bit of confidence going over the roof