All I could find in the Readme was a relative path used like symlink to make one pack show up in two folders. But is it possible to "symlink" resource packs not in this folder at all?

I have an empty folder that I wish to add mods' built-in resources to. For example, the mod support for Eating Animations is a resourcepack inside the mod.jar. I attempted to do so using the following reference:
"../mods/eating-animation-1.21+1.9.72.jar/resourcepacks/supporteatinganimation". (The .. is often used to navigate to a parent folder. The resourcepack in question is inside the mod jar as a resourcepack.

Using an absolute path (C:/Users/<user>/AppData/Roaming/com.modrinth.theseus/profiles/<modpack>/mods/eating-animation-1.21+1.9.72.jar/resourcepacks/supporteatinganimation) also doesn't seem to work. [I am using a custom instance in the Modrinth desktop app.]

There's currently no way to organise built-in packs I'm afraid. I'm still looking into ways to do that nicely. Loading packs from entirely outside the usual packs folder isn't something I've considered before, but could definitely be an interesting feature. I'll have to see if its feasible.

See #13 for tracking the built-in resource pack support.