The question is really "can I stand by for a more direct routing?"... and the answer is yes...domestic standby is free on UA and it is allowed on award tickets. If they are checking any bags they will need to let the checkin agent know that they want to go stand by so they can check the bags on the appropriate flights. There can be risks associated with doing this but generally if the route you are booking is A-B-C-D-E, and you stand by for A-C-E there shouldnt be a problem because you still have confirmed seats beyond C.

Where you can run into issues is if you are scheduled A-B-C-D-E and you try and fly A-F-E, where the connection point is different and you have no confirmed onward ticket from that connection point. So it's conceivable you could get stranded at that connection point until something opens up on a standby basis.

If you're booking these tickets with United miles I would see if you can confirm the convoluted routing and ask to waitlist for the more direct routing and it will automatically confirm if award space opens up. You won't be able to waitlist if using a partner airline's miles.