Is it possible to nail some nails or use screws to mount something directly on a slightly sloping felt roof? Or should you silicone first or solve it in another way?
Not if you want to avoid leakage. Silicone usually doesn't work very well on felt roofs either. Possibly roof compound like cold asphalt or similar. But it usually starts leaking eventually anyway. So that leaves "some other way."
I bought something from the tinsmith when we had a leak between the felt roof/tin hood that adhered to wet felt and tin at +/- 0 degrees, smelled awful and incredibly got a grip on wet surfaces. Maybe something like that could be used?? unfortunately, I can't remember what it was called 
check with the tinsmith.
check with the tinsmith.Hmmm, okay thank you.