lists are already spec’d. Would of course use those. We already do for mutes

You could do this already using a bookmark list (kind: 30001). Since each list item is just a tag and `t` tags are specifically for hashtags, this would work. Of course, clients would have to be looking for them for it to be very useful...

parameterized replaceable lists allow you to set a “d” tag corresponding to different lists while retaining the same kind.