DO NOT check-in for a flight you are going to miss. All you are doing is creating a scenario where gate agents are calling your name in vain to board. However they will offload you if you don't show at the gate.
Checking in for a flight you've missed in no way affects your status or ability to be put on a DM list for the next flight you desire. You would simply show up, state you've missed your previous flight and need to standby for the next one. By now you have accepted the risk of seats not being available but this can be pretty much determined by a phone call to UA reservations ahead of time.
Assuming you're on a restricted fare and things look real dicey or debatable about few seats being available, you can always opt to pay the $100 change fee for a confirmed seat on that next flight when you do get to the airport.