A related suggestion (which I personally think is a better way to handle it) would be to add a filter dropbox to the map window, so that one would be able to select from a list of types of icons. For simplicity, just use the types that Eloa has made in the POI submission database. (NPC, ZONE, MOB, etc)
Note that this would not be an active filter requiring SOE code changes, but a dropbox displaying predetermined (by the XML) groups of icons.
The only problem I can see with this is that if one wanted to show all the icons at once, it could mean redundant XML for an "All POIs" selection. Unless the XML is flexible enough to allow you to make a more active dropbox that can include multiple groups.
I hope that I've described my idea with enough detail here. I wish I had the knowhow to do this myself and post it as a contribution, but alas, all I can contribute is brainstorming power
