Bear in mind; step one would be having some sort of feature within XF to allow this. So really your support would need to be shown on a suggestion thread in the first instance then if it's something we add to the core we could consider enabling it here.
Why should they? It's a support forum and a bloody good one (Mainly @Brogan who answers my questions) - What you're paying for is something that works not a backlink and in the eyes of Google, it wouldn't be worth that much anyway, its a forum aka low quality (no offence Xenforo) - The only proper, valuable links would be from an admin, on the core pages really. Self made, clear cut user backlinks are crap 99% of the time and Google is well aware of that. You should know that being an SEO
Lets face it fella. This 'forum' is not the right audience to be spamming your dofollow links on - Even if you had the most amazing affiliate website on the internet, the audience here, are far more 'clued up' that the average web user, even if you got 400 hits from a link, your conversion rate would be shocking so all you'd be doing is actually wasting your time