To the best of my knowledge, UAL didn't have a JM clause at the time, so I'm afraid my anecdote doesn't directly apply to that question. That said, I know that in practice, schedulers work JMs into their projections as well. I'd recommend people take the same steps I did to avoid it when I worked at a company that used it. Know your contract. Be aware of what constitutes a "legal" JM assignment. Be acutely aware of the contractually acceptable means by which the company can notify you. Keep your union reps on speed dial (yes, even if they are useless). For extra credit, keep an emergency beer next to the phone at all times, to be opened prior to answering calls from unfamiliar numbers. If you DO get stuck with a legal JM, fly your contract and nothing more. If the resulting schedule disruption results in fatigue (REAL fatigue, not "I hate these arseholes" fatigue), notify scheduling of such.

A competent schedule head is aware that assigning a JM is a stop-gap measure, and indicative of endemic problems in the staffing level. An airline schedule head is likely to get the message eventually. Anything above and beyond your legally assigned schedule (over and open time) allows the required headcount to shrink. To do so with people on furlough prolongs their time on the street.