My feeling is that UA should have paid JAL for the cost of your extra bag, since you would have been allowed to check it for free on your original flight. Unfortunately, there's probably no easy way to get someone to authorize that when you're running for a replacement flight, and paying for it and trying to get the money back later would be VERY risky.

I was recently reaccomodated by UA from non-*A carriers after they got me to NRT too late to make my onward connection. When I got to my next transit point, I asked for lounge privileges, arguing that I would have been entitled to them had they gotten me onto my ticketed flight. The transit counter sent me to the TG lounge, where the attendant called UA, explained my situation, told them what my original flights were, and got their authorization to let me in (i.e., UA agreed to repay TG for my entry). So keeping 1P privileges when moved to a different airline is possible, but not easy...and I have no idea whether UA/*A would consider keeping those privileges to be an entitlement or a special favor.