For the vast majority of first come first served offers there will be no extra steps required by Patreon.
So in your example if you wanted to offer first dibs on a commission slot or other freebies to your patrons, that would be allowed by Patreon.
The only situation that I can imagine having a problem with is if it looks like a way to try and avoid the raffle rules. For example if you had a $1000 tier, and the only reward for that tier was that every month you gave out a prize on a first come first served basis that was worth thousands of dollars. That would be very suspicious and likely a violation of the spirit of our rules.