Syntax-aware tools always have issues when a team extends the base language to fit their problem. Rust has macros. People started using "go generate" for stuff like early generics. Does Mergiraf take EBNF or plugins or does a team fork it to explain their syntax?
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.
Since tree sitter parsers output a c library, you could dynamically load it.
The rust bindings themselves are a thin ffi wrapper.
If you wanted to make it a little smoother than needing to compile the tree sitter syntax you could compile/bundle grammars up with wasm so its sandboxed and cross platform
