I know this modded allows leaves to drop particles, I'm guessing its tag based?

Either way I'm wondering how you are getting that particle, are you generating it to just grabbing it from the particles associated with the block?

If its autogened I'd like to be able to use them in my resourcepack to add more thin leaf piles to immersive weathering, currently immersive just casts the leaf block to a 16x16x0 for the thin model using the direct texture from the leaf block.

If I could manually copy in 16x16 textures using a couple individual leaves generated by this mod I could get

Top row is manually made vanilla leaf piles

Lower left is color variants of oak, spruce, and birch

Lower right is some unique leaves

It could look ✨stunning✨ with me doing more handmade assets with the help of this mod

The assets aren't really dynamic but the particles chosen can be.

The particle textures Falling Leaves uses are found here and they are tinted by the average color of the leaf texture they come from.

Which particles are used depends on the per-block "Particle Implementation" setting.
(And also the general "Use Vanilla Textures" or "Always use Vanilla Particle Implementation" settings, but they are off by default.)
The "Vanilla" implementation will use vanilla particles or custom implementations provided by mods as well.
"Regular" uses falling_leaf_1 to falling_leaf_5.
"Conifer" uses falling_leaf_conifer_1 to falling_leaf_conifer_3 (though the spawn rate of conifers are also 0 by default).

By default, "Conifer" is used for these blocks, the rest of them use "Regular", except for Cherry, Pale Oak and Azelea, which use "Vanilla".

If a modded block provides custom particles, Falling Leaves will use them by default ("Particle Implementation" will show up as "Vanilla"), so you'd need to get the textures from that mod instead.

I hope it's not too complicated.
You can open the Falling Leaves config screen and after under "Leaf Block Settings" check what "Particle Implementation" says.

Oh I definitely have a lot more conifers than that, do I have to edit the code directly to update those things? Not in the config I see

also worth mentioning leaves don't fall atm because of a mod incompatibility Corosauce/WATUT#89

Oh, are you using the Forge version from CheaterPaul?
That version has nothing to do with this one (the official Falling Leaves).

In this version there's a config menu that looks like this:

I also don't believe they added any of the changes (like particle implementations) I talked about here, so you need to ask them on their page instead.