I do make a varnish by dissolving hard, brittle pine pitch in alcohol to seal primitive arrows and sinew wraps on arrows. I guess it actually isn't a true varnish because it doesn't have oil in it but it works well to seal arrows and sinew wraps. I don't think I would use it on bows though.
I have finished a few bows with hand rubbed bear or pork fat and it worked well.
Animal fat, bees wax and tree pitch blend together well with a little heat but be careful because the pitch in very volatile so don't overheat it. Different mix ratios will yield different consistencies for different applications.