End of line after socat readbytes limit is reached

We wanted to use socat to make a process available that is line-based:

socat SYSTEM:"echo \$\$;sed -u /^%/s/foo/bar/" \
  TCP-L:1234,fork,reuseaddr,readbytes=21,bind=localhost

This works but only until the readbytes limit is reached because the last "line" is cut short and the left side ("SYSTEM") does not get a newline until the next line. For example:

$ echo %foo | socat - tcp:localhost:1234
13047
%bar
$ echo foo w/o ^% | socat - tcp:localhost:1234
foo w/o ^%
$ echo %foo, here is a too long line | socat - tcp:localhost:1234
$ echo %foo | socat - tcp:localhost:1234
%bar, here is a too l%foo

The echo \$\$ is there to demonstrate that socat starts the left side only once and keeps it running for multiple clients. That's what we need. But is there a way to e.g. pass a newline to the left side when the right side was cut short due to readbytes?

Update 2019-03-15 00:05 UTC:

One will probably want to add ,pty,rawer to the left side, however, this does not help with the case where the readbytes limit is reached.

socat is 1.7.3.1, running on Debian 9, Linux 4.9.144.