I used masonite panels a lot in the early days (the smooth side, not the bumpy side!). It works very well and there is no need to treat them before working in encaustic. Given the surface is dark brown, you can adhere paper to the surface as long as the paper is at least 140 lb weight (140 lb weight refers to a ream, which is 500 sheets of paper). I've used 140lb drawing, printmaking or watercolor papers adhered to masonite or panel and successfully worked with encaustic paint. If the paper is too thin, I would be concerned that the adhesive might soak through the paper. I would use either Lineco pH neutral adhesive OR an acrylic gel medium. Make sure you apply enough adhesive and put weights on the surface over nite to make sure the paper is adhered completely.
If you wish to have a white or light colored surface, you could also apply a few coats of white or light colored encaustic paint, fusing in between layers, as an underpainting!