The big question: if Boeing does this, would they be a headhunter middleman, or would these be regular Boeing-factory acceptance/test-pilots?

If these are regular Boeing guys, they are unlikely to feel scrutiny or pressure due to their scabbish actions. (Since not pursuing a 121 job, but would also be in a position where they could not say “not gonna do it”).

If Boeing is just the crew-placement service, then they will likely get the vermin one would expect...and protests will fall on deaf ears.

If UA is doing a joint venture with Avianca (I am unfamiliar with the details), then the strongest position may well be UA-ALPa telling the company we see this as a strike-breaking move (after the fact).