You wouldn't be able to get quite as fine-grained. One memory per object is probably horrifically slow. And I don't know about Fil-C, but CHERI at least allows capabilities (pointers with bounds) to overlap and subset each other. I.e. you could allocate an arena and get a capability for that, and then allocate an object inside that arena and get a smaller capability for that, and then get a pointer to a field in that object and get capability just for that field.

Fil-C has like one "linear memory" per object and each capability gives read/write access to the whole object.

But Fil-C has its compiler which does analysis passes for eliding bounds-checks where they are not needed, and I think it could theoretically do a better job at that than a WASM compiler with multi-memories, because C source code could contain more information. Unlike WASM, but like CHERI, every pointer in memory is also tagged, and would lose its pointer status if overwritten by an integer, so it is still more memory-safe in that way.

One would probably just need to define WASM extensions that allow for such subsetting. Performance will probably be competitive with software implementations of CHERI (perhaps with varying levels of hardware acceleration down the road) which isn't that bad.