I am not a developer and admin so I have not run a signature campaign before, but if a project run a signature campaign in any forum the thread about the campaign will create back links and popularity on the project which is good for the project's reputation in the internet, that is why doing a campaign can make a difference in short and long term.
First of all, sorry for very late reply.
Link from ANN thread or sig campaign thread means nothing at first and it's a matter of time whether that thread will become popular or build backlinks from another websites, after that, that single backlink from ANN or Sig thread may have some value.
But what I talk here is about following: When you hire posters on this forum, it happens like some of them have already posted in threads that have high UR (Url Rating). That backlink can be high quality as UR is higher but at the same time it depends on how much referring domains that threads carry.
Whatever Bitcointalk as a high domain authority webpage can be very helpful with improving SEO score, but what after the signature campaign ends and users change his signature?
It is okay but still, I think, links in the contest have a much higher score based on SEO.
Links are lost after that and the domain authority that you got from backlinks from your participants' posts will be gone. It will be interesting to check ahrefs score on websites that had a lot signature campaign participants and ended campaign after certain time. I'll check it but from personal experience, was only carrying one website on my sig for some days, it improved website's score by 0.3 Means nothing significant but only some links were indexed.