Their system will only allow you the long haul allowance on a domestic sector if it is part of the same ticket - it is the ticket, and not the PNR, which carries the luggage allowance.

Some airlines have policies where if you’ve just arrived off/are connecting to a long haul flight with a partner airline they will respect that allowance but it’s at their discretion. If the flights in question are not direct connections off or onto the long haul sectors I don’t think you’ll have much luck.

That said, the additional bag should be quite cheap.