Short answer: Yes. Mustang's jhat works against heap dumps produced from earlier JDKs as well. It just works -- because it uses the same HPROF binary file format as used by hprof profiler and jmap tool. jmap tool can dump from live Java processes and Java core dumps. This feature has been backported to 1.4.2_09 (solaris only) and 5.0 update 5 (solaris and linux only) releases. hprof produces heap dump with allocation site trace information in the same format -- you need to use format=b option with hprof. As you'd probably know, hprof prodifier has been existing in JDK for quite some time.
And yes, HPROF binary file format is platform independent. Therefore jhat on any platform can parse and view heap dumps produced from any platform