I have found that across the major airlines, US check in and lounge agents seem pretty experienced in IRROPS and generally will consider many options. I was in SFO on a DL/VA itinerary (SFO-LAX-SYD) and the SFO-LAX flights were all messed up due to weather and it didn't seem there was a chance of making the LAX-SYD flight. After trying various things, the DL lounge agent endorsed my ticket to Qantas and I flew nonstop SFO-SYD instead. This required some kind of special authorisation from VA but the lounge attendant diligently followed it up, despite being very busy with plenty of other people needing fixes. I did have to collect my already-checked bag and then go to QF check-in to get them re-tagged, but that process was efficient.
Getting more on topic, on another trip I was flying JFK-SFO-SYD on AA/QF, and the JFK-SFO leg was delayed for some reason, and they managed to rebook me JFK-LAX-SYD with almost no fuss.
Maybe I've just had good luck but I find the agents really try hard and have a lot of tools available to them to make you happy and get there on time.