knit-in ,
While doing something like this violates the airline rules and your contract of carriage, it is not illegal. E.g. it is a civil matter between you and the airline, and criminal charges would not be brought. Further, I'm not sure the courts always go with the airlines interpretation of the contract.

On a more practical note, indeed, your reservation would cancel because of the no show. If the gate agent was either very busy or sympathetic, they might let you on. You all know how crazy airports can be, and they simply don't have time to spend ten minutes sifting through the history of your pnr to figure out what happened, they just have to get that flight off the gate five minutes ago or its really going to screw up the schedule for the rest of the day.

On the other hand, you might hit a quiet gate, and a gate agent who is intent on sifting through your record for forty five minutes to figure out exactly what happened, and how exactly you are where you are with your reservation showing like it is.

It is a gamble indeed not without risk or not for the faint of heart.

I would not call CO reservations but instead try to get an airport agent to fix your record so the segments not flown are cancelled. Like maybe you would say you had a connection in IAH where you want to start out, etc. So the next time you are flying, if you see an agent you think is nice, see if they will do it for you. Only trouble is you would have had to buy the ticket. You won't just be able to go up to the counter, and say oh yah I want to buy this hidden city fare.