Lee, after reading the entire ruling with respect to Delta's pilots working overtime - the appeal, not the original verdict - I believe that the current legal system will consider mass voluntary resignations to be a coordinated job action in violation of the RLA.
Over the last 15 years, the chips have become heavily stacked against labor unions that fall under the RLA. We are now threading a very small needle on the actions that we can take.

With that in mind, unions need to reconsider how contracts are constructed. Management can go in slowdown mode, dragging out negotiations for years. Rather than attempt to get retro pay, I think all new union contracts should have automatic payraises built into them at the amenable date. For instance, an automatic pay raise of CPI-W plus 3% annually until a new contract is signed. This would eliminate the need for retro pay. Also, it's crucial that the figure be tied to a CPI metric rather than putting a finite number such as 4% annual pay raises. We have no idea if we'll be in a low or high inflationary environment when the amenable date occurs.
Just imagine putting in a clause where we get a 4% pay raise annually at the amenable date but the inflation rate is running at 8%+ when we reach the amenable date. Management would have even greater incentive to drag out contract negotiations.