3 weeks of conversations later, I was told that the 18 month rule was "fair" in everyone's opinion, and that if I'm really concerned about working on projects that are interesting to me, I should just find one to spend my 20% time on (easier said than done).
If you approach situations like this in a constructive and professional manner, the company will go to great lengths to find a solution. At least that's been my experience.
i had potential teams lined up (one of which I would've been allocated to initially, had they not hit their new grad hire cap), so it was certainly a very frustrating experience. perhaps they care more about established engineers than recent graduates?
This is the best way to align everyone's expectations.