I'll try to look into this in a moment.

If we do have to make new surface indices, we would also need a way of routing those such that they play the sound of any of the existing surface indices.
For example, say we use 50 as an index, which will be a variation of dirt with different particles. We would want it to play the same sound as the dirt (index 3), while still being index 50.

I'm thinking this will be declared on the same level as the ForegroundTiles.xml, so it'll only affect the maps that use it.

As for the particles, maybe we could allow copying from either Dust or SparkyDust and modifying the colours and other parameters from there? That seems like a simple solution for the time being.

By the way, I've found a line of code in Player.CreateWallSlideParticles that references the returned value from the dust particles check, and uses it to check if it's equal to Dust, in order to determine what I'm assuming to be an X offset from the player's center where the dust would burst from.
This might be worth using as well.