No unfortunately the OP cannot check in at AC or UA desk in SFO for an SEA departure.

Correct SEA is not listed as accepting OLCI from AC. This is because AC flights out of SEA use the UA departure control systems rather than AC. From an AC perspective this station is not DCS enabled.

If your tight and have no checked luggage, I would proceed from your AS arrival directly to the AC gate and check in from the gate. Once you explain to the gate agent how you got there (might be a good idea to hvae your AS BP handy), then UA agent for the AC flight should be able to help you out.

The big wrinkle in the above recommendation is that I am not familiar with UA DCS application and its features. With AC DCS, the gate agent can check in after the cut off period (I have had it done a few times when I get to checkin a few minutes after cut off but with no checked luggage).