Do I think mass thread flooding would occur? No. Do I think their would be an increase in stupid shit posted by individuals simply wanting the rank? Yes. People like ranks and they're cool to have. If I didn't meet the requirement and this was a few years ago, you can bet I would be posting more to meet such a requirement.
You did not clarify that individuals made their own ranks, rather you stated they chose what was in it. The problem rooted in self-rank creation is the content will not be equivocal across the board. Making ranks isn't hard, I've made a few of the mini ranks myself; however, little things like not having a border will disparage the looks of the entire user interface. I realize it's the spam forum, but there's no need to the user interface look bad. All of this can be avoided with a simple universal fun rank, or maybe 2-3 alternatives that can be selected depending on the method of handing out the rank that is chosen. Avoiding even the UI issues, there's still the manner of the ranks themselves being applied. This application process is significantly extended when there's a high amount of custom ranks.
I think you misunderstand what the concept of it being a pain is. The spam forum could be the least active section, it still doesn't change the steps needed to apply the rank to each individual that requests one alongside the actual upload of the rank. One time? Not very time consuming at all, but multiple times? It gets tedious fast and continually doing it would be a hassle. I assure you there are better things for Admins to doing and even if there wasn't, I don't imagine they want to do. I know I wouldn't.
I thought of this, but I would be against it. If a fun rank is being given it out it shouldn't be to a random user within the spam forum itself; rather, individuals that have used the spam forum continually.