When you put it that way, I kind of like this idea even more now.

Haha, that was sort of obvious.
I'm a bit torn between chairs. On one hand (aka chair) I find PowerGlove's idea quite compelling, on the other hand there're (not very strong) reasons for me to not like it and I don't think I'm trying to cling to the status quo. If there were an easy solution, we likely would've it here already.
Low ranked shitposters can still shitpost. I don't see serious limitations for them here, especially when there's no newbie jail. And theymos stated quite clearly there will be no newbie jail.
Is this only about trying to prevent that shitposters can rank up and be eligible for signature campaigns? There might be other, more suitable metrics to sieve out or prevent shitposters to enter paid campaigns, like merits earned in low, mid, long-term periods, merits-per-post ratio e.g. And campaign managers have a responsibility, too, who they accept into campaigns.
For me to decide if I want to put someone on my ignore list, I consider their content and merits-per-post ratio. Someone with a low ratio, say below 0.1, is way more likely to be ignored in the future than someone with a higher ratio who just happened to annoy me with their current crap in a particular forum spot.
However, most forum members don't post solely on one board. And again, we're talking 1 earned merit for every 10 posts. This shouldn't be too hard for anyone to do. If it already is right now, someone can't earn 1 merit in 10 posts should seriously question what it is they are actually doing here.
Actually I can imagine who will it affect the most: bounty hunters. There are hundreds of accounts with less than 5 merits and thousands of posts. They will quickly go from Jr. Member to Newbie. Maybe some boards could be carry-exempt, like Bounties, Dev & Tech Discussion, and Serious Discussion.
I get that and am actually quite on your side here, to be frank.
How about a new rank "Shitposter" that is skipped when ranking up, i.e. can't be reached by "normally" ranking up? But you can be de-ranked into it! This rank, once reached by enough shitposting with PowerGlove's proposal, would put severe limitations for those who become de-ranked into it. The details of such limitations are tbd.
Users with rank "Shitposter" will likely simply abandon their account? Not sure about that.
I would be fine with a month temporary ban for an account who gets de-ranked into "Shitposter". On second occasion, temporary ban of 3 months, increasing by 3 months every time.
Ah but they often have "friends" that give them merits for their shitposts, enough to rank up, anyway.
Mystery merits economy...