Whilst visiting a mid 18th century high status home turned into a local museum in Middle Pennsylvania in the early 2000's, I bought a soft cover book that had original formulae and documented information on 17th/18th/early 19th century furniture finishes. The author even commented on how it seemed there was no such thing as plagiarism when copying parts or all of earlier formulae into later treatises and not giving credit to the original authors when known.
In late 17th/early 18th century treatises, it talked about using shellac as what we might call a base coat on a furniture finish, but for a different purpose than mentioned above, for just filling the pores of the wood. Instead, it was also used before logwood or other period applied stains were used on furniture wood to "color" all the wood to the same matching color/finish. This was especially important when using expensive imported mahogany lumber, where not all the boards would match in color near the middle to end of the imported quantity of lumber. The Shellac base coat kept the stain from being sucked into the wood pores at different rates and thus to different shades of color after staining. IOW, the shellac base coat "evened out" how the period applied stains would be absorbed and color more evenly over different pieces of wood. I tried that tip when staining modern stocks with one or two handguards, to get both or all three pieces of wood to match, though I used Tru Oil instead of Shellac. I found I needed to sand the top surface again before applying the stain and then top finish coats, but it worked beautifully to get all three pieces of wood to match.
Bottom line, there is plenty of period documentation for using shellac (or something else) for a filler before applying the varnish over the top.
BTW, Birchwood Casey's Tru Oil is an Oil/Varnish finish that does not need to be prepared as done in the 18th century. Yes, there were stock finishes in the 18th century that came out as shiny as Tru Oil, but if one doesn't want that shiny of finish, it is no problem to use abrasive pads to "knock down" the shine and when buffed with a terrycloth rag or towel, you get a sort of "warm glow" looking finish.
Gus