IBM Alphaworks has released (4/24/2006) version 1.2.9 of an MPEG-4 player and content creator development kit that works with Java 1.3.1. Reportedly, it plays MPEG4 video (samples included) just fine on a PII-300MHz machine.
http://www.alphaworks.ibm.com/tech/tk4mpeg4
Source: OS/2 Warp News and Rumors