Well, there's option of setting 5- or 6-string bass as a requirement when submitting customs to the database but I think Ignition, the current database engine, isn't able to actually filter or even show that information, so there's no way to look for it atm. There'll be an update eventually which should make that possible, at least that's what I hope. Not sure if anyone's made a list of songs that require 5- or 6-string basses, but you can search for it. I also don't know what genres you are interested in and apart from the one's I made, I don't really have a lot of info about that. A lot of progressive rock/metal bands use 5/6 string basses (Dream Theater, Haken, etc.) and also many modern musicals require basses with more than 4 strings (I released a few customs from musicals like that). Making a list of my own stuff will take a while, so I'll only do it once I know what you are really looking for.