The way I did l tags in coracle (initially) is actually purely as an attribution of t tags to other people's content. So ["l", "topic", "#t"] says the referenced content should have #topic associated in the author's opinion. So I think you could use lists with t tags, 1985's with t tags, and t tag follows in a unified way. I didn't see a need to differentiate in my case between topics and "ugc" since they're basically the same thing.