I have done this before. I was flying from SCE-IAD-FRA. Once the delay was posted for the IAD-SCE (which will subsequently cause my SCE-IAD to be delayed), I called UA immediately and they removed the SCE-IAD segment, reissued the ticket, and then we started driving to IAD.
I think once you have a strong case that you are going to be in a misconnect situation, I think you can plead it to a phone/airport agent to remove the segment. Then, you can on your own to get to your new origin.
In my case, the ticket was purchased from LH so it was not a 016 ticket. But because of UA's delay, they had to take control of the ticket and reissued it into 016 stock.
