I think I saw a different mod (some version of Genesis?) do that by having a couple different leather templates- there was your ho-hum regular leather, but megabeasts etc. produced stronger and more valuable "tough leather".
Yeah, that's how it'd have to be done.
I'm just starting to look at mods, and I'm curious why you need to specify different versions for different graphic packs.
Some graphical information is stored in the raw files - most notably all plant graphics, all rock graphics, all vermin graphics, some workshop graphics, most tool graphics, and most color information. Exception: the newest, TWBT-dependent graphics packs replace even these, but they still need raw changes to be able to distinguish between e.g. wheat and papyrus.
Another thing a lot of graphics sets like to do is remove diacritics from the words in the language files to free those symbols up for closer representations of their other uses. For example, ö is used for minecarts and wheelbarrows, but also appears in a number of words. If you don't replace ö with o in the language files, you're limited to graphics for minecarts and wheelbarrows that look enough like 'ö' that names still look like names, and not strings of characters with a minecart in.
I do have a utility for applying all graphical information to a set of raws without destroying any mod-only information - which you can find in my signature, if you're willing to check out the code and run it unpackaged - but it's not polished enough to expect any old Joe to use it yet. I do use it to create the Modest Mod's graphics versions, but a) I don't need directions since I wrote it; b) I don't need a GUI; and c) I want to be able to doublecheck the results before publishing.