Yeah at the moment it just supports whatever the tree-sitter parser accepts, period. A bring-your-own-grammar version could be interesting, I don't see why it couldn't work. Do you have any Rust crates to recommend, to do parsing according to a grammar supplied by the user at run time? It's likely to be slower, but maybe not prohibitively so…

Another approach would be for the tool to accept doing structured merging even if there are error nodes in the parsed tree. If those error span the parts of the file where the extended language is used, then the tool could still help with merging the other parts, treating the errors as atomic blocks. I'd be a bit reluctant to do that, because there could be errors for all sorts of other reasons.