I think this should be abstracted to allowing the chiseling of non cube block models.

Maybe this could be implemented with a material system, where e.g. cobblestone blocks, stairs and slabs all drop the same "Cobblestone Bits". This might cause issues with textures when converting between regular blocks and bit collection blocks, however

Oh the block connections would have to be specified manually for it to work, probably. Could maybe be done with new datapack tags?

The texture issue probably doesn't matter too much; if you're using mods like Better Grass or Connectivity, there's already noticeable texture changes when you convert a block into a bit collection block. Seems like the sort of thing that might not be worth fixing.

This is an interesting idea, although I am not sure how you would associate say cobblestone stairs to the cobblestone block. The only connections I'm aware of between the two would be the ID (but basing it on ID wouldn't be smart as there is no way to enforce any pattern), and potentially the fabricblocksettings, but I am not sure how that would work for vanilla blocks. In regards to the texture issue it would enforce that for example stairs and normal block to use the same exact texture, or else you could just enforce the one being use by the block if the block has been chiseled, but I don't think thats ideal.

I hadn't thought of that, what if Bits and Chisels was data driven and so only is able to chisel blocks that are designated a material using tags ? The only drawback to that is non-immediate functionality with other mods (unless they explicitly add the necessary tags). Honestly the main use (of which I can think of) of a material system is the lack of duplicate bits, which I wouldn't say is an immediate issue. Also for the code side of implementing non-cube block models chiseling maybe you could just consider it as any other model which has had its bits altered, I think it would be quite feasible.