I have seen references of colored varnish ( did they mean paint?) used to protect (
or maybe decorate???) leather covered carriages in the 18th century...but for decorative purposes "guilding" was more common...like for traditional painted wall coverings...but I have not seen references where leather was ever commonly protected with varnish
of any sort. heavily waxed...yes...dubbined...yes...olied...yes...pitch lined...of course, but varnished...not yet.
It seems once you seal the leather with varnish, oiling the leather again when it dried out would be problematical...also, as shelac was the traditional "varnish" of choice, and not too weather/water durable,
I would think varnishing common long rifle accoutrements was not often done...and if it were I would have thought it would have become more prevalent the more commonnly available various varnish concoctions became...to the point where it was seen alot today, both on modern work and vintage leather goods...
Perhaps portmanteaus...or hardened armor type leathers from the 17th century employed it? But I havent seen it described yet....not saying it wasnt...just that I havent encountered it yet for what ever thats worth...but I too would love to see documentation..it actually sounds pretty interesting.