However, this list at least provides a source for the ingredients that we can show were in fact used (even if not widely) at some point in period for leather dyes.
I haven't done any deep research into the less common of these to figure out exactly what they are (for example, I know about murex and lac but not sure what "murex lac" is). Some of these ingredients (like the murex lac) are used in making compound pigments, rather than as dyes on their own.
I'm pulling this information together as interesting detail for the documentation of my Fall A&S project for the Queen's Prize Tourney, but thought I should post it here as it could be useful for other people as well.
Black grape skins
Grains of Narprum (may be Buckthorn berries)
Kermes (also referred to as vermillion)
Litharge of gold
Lulax (compound pigment)
Raphanitis/Ireos flowers (Iris flowers)
Reno citrino (no idea what this is)
Ripe Buckthorn berries
Verzino (compound pigment)
Walworte (also referred to as elderberries)