Something like this is what I've suggested for firehose-style RSS-feed operators (e.g., academic publishers, arXiv, etc. with very high volumes and typically predefined yet overlapping categories) but to no avail.
Cool idea, I have some of arXiv's feeds in my own list for this. Could also definitely hook this up to the HN firehose feed, I'll try that..
That's a nice project, I'm interested to take a look at the source code if you publish it!