While I would suggest a complaint to UA, there are actually no requirements, IIRC, with respect to having a certain number of or any toilets on board, so not sure what the justification of a DOT complaint is. In the past several years, I have been on a couple of flights, regional jets, with inop lavatories. But GA has announced prior to boarding to allow people time to go beforehand, and these were relatively short flights (under 1.25 hours). One time, I was on a US Air regional jet, think it was a CR2 but might have been a CR7, where we were on our way out, and stopped on the runway as their was some sort of issue. When we pulled up to the gate again, I asked if I could use the Lav, and then on my up, was told no because they forgot to load 'blue juice' - but they would get the rampers to do it since we were at the gate. Turns out they emptied it from the toilet the night before since it was cold so it wouldn't freeze, but forgot to load more in the morning.