Yes. You are right. At the moment there's no exposed functions for altering that binary data structure.
Because of the way how Postgres works though, the row will always have to be rewritten in the future (all updates to a row will cause a new copy to be written - rows are immutable in Postgres). What we might gain in the future is a way to skip the parsing process, but the document will always have to be rewritten.
Do writes still require loading the field into memory, parsing, updating and the writing back?