I occasionally use RC paper as a positive contrast reduction mask when exposing photographs by contact. The sandwich in the contact frame (counting from the glass down) consists of a RC contact positive face up, the negative used to make it, and a FB receiving sheet. Exposure doesn't seem to be overly long because RC paper lets through a fair bit of light. The RC base diffuses the positive image to yield desirable "unsharp masking".

Registration is no problem. I just shove everything into the same corner of the contact frame and all the sheets line up automatically. By the way making positive masks out of FB paper does not work. FB changes size when processed!