I've added some code to the jack_transport external (tb/jack_transport/ in pd-extended) to show the current bar/beat (and also bpm and tempo) in real time. Messages are only sent out when pd is computing audio, and also no more than once per beat (to avoid floods of messages).
I'm not outputting all the available information from the jack transport -- e.g. the current tick within the beat -- either because it updates too quickly or because it's just not useful to me, but it would be trivial to add those.
The motivation here is that I'm using klick -- http://das.nasophon.de/klick/ -- as the jack transport master (mainly to control sooperlooper), and now it is possible to trigger events within pd at particular beats or bar numbers.
The code is available at http://artdent.homelinux.net/svn/jack_transport~/ . The makefile should be suitable for building jack_transport~ from inside a directory that is sibling to the existing jack_transport. I've only tested it on Linux, but it should work anywhere jack_transport does.
I hope this external is useful to others. Suggestions for improvements are of course welcome.