Sorry for the late reply - I had some trouble with the new forum software.
You can send a message to a user tty (by using /dev/tty...), but there is no additional support for sending characters to serial ports. There is no functionality to receive from serial ports. This is also not on the todo list as there are many other things, in much greater demand, already on the list. If you absolutely need it, you may consider funding the development of such a feature:
http://www.adiscon.com/Common/en/Products/techsup.phpFor the heartbeat, I think the solution is the immark module. It generates -- MARK -- messages, originally thought of being written to a file. However, I don't see any reason why the should not be sent to a remote host. So I would go and have a look into that module.
I hope this answers your questions, if not, keep posting

Rainer