As far as I know, you can write in anyone for your Tier 3 RSP GM Boon. If you want to offer it as a trade option for another boon, I don't think that violates the spirit of the boon.
Thanks a lot. Will offer it as soon as it's available (5 more games!).
Is it possible to offer writing your name on my Regional Support Program GM Incentive #2 as a trade? Asked on Discord and they were unsure. So far I saw Hmm offering such trade but I want to be sure it's legal.