Tvheadend

What you're asking doesn't make much sense (at least to me).

You mention a "client". What client? There are dozens of clients that play tvh content!
And then even if tvh was enhanced with what you have in mind the "clients" would need to be enhanced also...

By the way, if the song name is fetched from the mp3 stream (like you mention) then doesn't it make more sense to let the client do it (whatever client it is).

Anyway, for xmltv, tvheadend already allows you to run a grabber script crontab style - for example you can run an epg grabber script every 1 minute.
The grabber script can do whatever you need to fetch the song name and feed it back to tvh.

EPG is not RDS

It maybe better to parse the mp3 metadata directly and update the EPG database. Also, don't forget that we have 'external xmltv grabbers', so you can just feed xml to a named pipe and this will update the EPG database, too.