Tbh I'd rather have some way to signpost and give greater prominence to the Petitions board in general. I'm 100% behind this one, and it's time-sensitive - but there are often other petitions with equal or perhaps even greater importance. (Just take a look at the first few at the moment!). I can't see MNHQ wanting to pin petitions unless they relate to one of their own campaigns.
One thing which perhaps could be allowed within the general rules is for boards which may have a particular interest in certain petitions to have a 'check out those petitions' type of thread, with links to the threads on the petitions board.
(That sounds more convoluted than it really is). That might serve the purpose of promoting issues of interest to specific groups without risk of 'spamming' .