Originally Posted by Phteven

There is legal precedent for this, and when this has been pursued by company managers in the past they have not succeeded in getting restraining orders or injunctions prohibiting pilots from denying overtime. Delta tried this back in 2000 and the court said that though they found evidence of illegal job action, the allegations were insufficient and too vague.

Delta Air Lines, Inc. v. Air Line Pilots Ass'n, Intern., 123 F. Supp. 2d 1356 (N.D. Ga. 2000) :: Justia

Regarding your question:

"There is no question that there is an ongoing concerted effort on the part of some Delta pilots to refuse overtime work and this Court recognizes that a campaign to ban overtime work or a concerted refusal to work overtime actively sponsored or promoted by a union may violate the RLA."

They go on to explain that the issue is a lack of sufficient evidence.

In short, a blatant "we won't pick up overtime until we have a contract" would almost certainly be construed as illegal, especially if ALPA openly endorsed it. However, it has been done before without court action.