The structure for the write head logs needs to different between systems. For Postgres, the WAL is a record of writes with new blocks. It can't be used without knowing the Postgres disk format. I don't think it can be used to construct logical changes.

Using a standard format, converting things into logical data, would be significantly slower. It is important that WAL be fast because it is the bottleneck in transactions. It would make more sense to have a separate change streaming service.