A letter or other notice to the plan is not going to provide any protection unless the plan has decided that something less than a domestic relations order will suffice. The law requires a domestic relations order before a plan is required to act (including to protect a would-be alternate payee). If the plans are based in California (and maybe elsewhere) they will probably treat a California joinder order as a domestic relations order for purposes of preserving benefits until matters are resolved. A joinder is an abomination, but most plans will choke it down with the intended effect.