Unlike other opinions here I do think it is technically feasible to stream a copy of the WAL - it just has to be implemented in the VFS. "Shared memory" could be a SharedArrayBuffer, or just a normal buffer if you only have one database connection open at a time (for example in a SharedWorker, which is already common). It may not be simple to implement, but definitely possible.
The biggest actual issue is that it will capture block-level changes, not row-level changes. This means it can work to replicate a complete database, but partial sync (e.g. sharing some rows with other users) won't be feasible.
To get row-level changes for partial sync, you need to use something like triggers or the SQLite session extension [1]. For PowerSync we just use triggers on the client side. I find that works really well, and I haven't found any real downsides to that except perhaps for the work of maintaining the triggers.
[1]: https://sqlite.org/sessionintro.html