The better solution is allow control over what the terminating characters are. Rust raw strings allow N ‘#’ characters around them, like r”x”, r#”x”, r#####”x with “quotes””#####. So no escapes are ever needed, just increase the number of hashes to outstrip the maximum consecutive hashes appearing in the string after a quote character.
I’m curious what you mean by this — why does coding for length prevent streaming? The receiving end can certainly treat the text as a stream still. Do you mean that the sender cannot “stream” if they are generating the contents on the fly? That seems trivial to solve - just break it up into chunks, assuming a little bit of buffering is OK.