Configurable message length limit for HTTP API

We currently accept all (valid) outbound messages in the HTTP API. Ideally, we'd like to reject messages that can't be sent for some reason. Since we don't (currently) have any way to get capability information from channels, the easiest thing to do here is add conversation config, starting with the length limit.
If I'm understanding this correctly, both send and reply messages uses the same config key for message limits. Would we ever want one to have a different limit to the other?