Thanks for your update.
We have no current infrastructure for hybrid classifiers which use more data than the low-level features that we have in AcousticBrainz, although this is a great idea which could be interesting to try and fit into our system.
Last year, we had a student work with us for Summer of Code (https://community.metabrainz.org/t/a-job-running-client-for-acousticbrainz/7003), though we didn't get the whole project finished. This would be a good start to a system which lets people write their own classifiers - although we would have to work out the split between training (where we take a dataset and end up with an accuracy percentage) and computation (where we take new submissions and calculate a feature - would we distribute this back to the external contributor's server, or somehow run it on the AB server?)
With the other things that we want to do with AcousticBrainz this year, it's unlikely that this will be a focus for us, but if this direction interests you, let me know and we can see how we can work together.