I have always been a fan of the adagium 'lurk moar', so I think that that is a good idea. Spam is mostly linkspam anyway, I doubt there is much comment spam around. It might be worth keeping an eye on what kind of accounts promote what kind of accounts, because you might have someone spending some actual time here and then just promoting all of their bot accounts. Maybe to continue the 'hubski as a bar' analogy, you could use the word 'welcome'? I imagine a bar regular welcoming a young padawan to the table.
I'm on the other side of this fence. If there is a great comment thread with several really good ideas, or several posts with excellent comments, I don't want to be told I cannot reward that person for their quality content due to some arbitrary constraint imposed by the software. Participation should not be punished. However, I could see limiting a user's number of promotions for their first month of membership, or preventing them from being the first person to promote a post... that could limit the spammers enough to discourage them. I am starting to like the idea of rate-limiting promotion to once per person per day.
This may be too much complexity, but we could allow you a daily amount of "welcomes" based on how many badges you have. If you only have one badge, you get one welcome. If you're kleinbl00, you get a thousand.