The paper underneath the gelatin is itself white and reflects quite a bit of light: just think how different the projected image looks on paper vs. on a black easel. I'd guess that any effect of light going all the way through the paper, being reflected by the easel, then all the way back through the paper again, would be very subtle .. probably like adding a tiny amount of diffusion, which would make the shadow areas "glow" or softly spread.
I make pinhole cameras out of tin cans, and I use photopaper to make the negatives. The inside of the cans are usually shiny metal, or colored white, and I usually spray paint the interior of the can flat black, mostly to avoid reflections from the parts that are not covered by photopaper. One time, out of curiosity, instead of painting the inside black, I left shiny metal behind the paper, and taped 120 black backing paper over the parts of the surface that would not be covered by the paper negative. I
thought I could see a subtle difference in the results in general ( a sort of softness ), but when I made a solargraph it was obvious: the streaks left on the negative by the sun did not have crisp edges. So my hunch is that reflections from underneath your paper might affect the print in a tiny, almost invisible way, and it would make the most difference if you have something like a "specular shadow" in your print;: a small area of perfect black surrounded by a bright area. In that case, the edge of the shadow might "bleed" a little or be slightly soft. Although you'd probably have to look at your print with a magnifying glass to notice it
