[Egregious Jr.]

Basically, there's someone with the username 'zachary', who's closed and has almost no posts. Is it against the rules for me to use invisible characters to copy this username?

[Acceptable]

No it's not against the rules as long as you're not impersonating him. Which I can tell that you don't plan on doing. Keep in mind though that special characters can make it harder for other members to find you on the members list.

[♥♥♥]

I'd say you'll be fine unless there's a rule regarding invisible characters in your user. Also make sure that whoever was closed with that name didn't have a bad rep here.