I got inspired to play with the HRM belt again. The most important field was actually quite easy to find, but there are some fields that are still a mystery. Anyway, my findings for now are documented in a github repository, with some data dumps if you like to help out decoding the mystery fields.
I also made a simple J2ME application for testing, it even worked on the first try, damn I'm proud. I'll post that to github too after some minor cleanups.