if you don't want to super look like you're doing this, comment to other people you're interested in playing with first, and then toplevel comment with most of your plans already set, and one or two specific interesting plot things that you want from your remaining open slot, not just "I have room for X threads" but "I've got A who's going to attack my character and B who's going to rescue them. I'm interested in someone who might encourage them to take the stupid risk that leads to the attack in the first place. Idealists and Iagos both welcome."

not every shit tier will be thrown off by "hey you actually need to carry this character motivating thread" but many will. if the thread is dull you've still got other fun plans for the event, and over time you'll get some good new CR out of it/not be totally cliquey, while still limiting the bulk of your play and limited time to the people you actually want to play with.