During a decade of flying QF 737's I never once had a Captain suggest we fly an aircraft from a non maintenance port without disclosing a problem with the aircraft. I have, however been grounded in non maintenance ports on many occasions due to U/S aircraft. On any of the occasions where maintenance watch were contacted, we would always ask for any advice received to be put in writing and forwarded to us prior to departing and only when we were completely satisfied all actions were within the rules.
I have flown with approx 150 QF Short haul Captains & have never seen the rules bent as you are suggesting.
If cowboys represent 1 per cent of the pilot body (I haven't seen them), he would need to be paired with another 1% FO who agrees to break the rules. Then also operate through a non maintenance port & have a defect. What are the chances?