My understanding is yes, probationary pilots who honor the picket line could immediately be fired.
Additionally, probationary pilots do NOT have access to the grievance process for terminations. In other words, if you get fired on probation, there's nothing the union can do to get your job back except ask really, really nicely.
In this case, the MEC has two options:
1. Ask the probationary pilots to go ahead and fly trips assigned by the company. The company will have it's 10 percenters (senior captains who will vote against a strike and who will cross the picket line) so that's about 160 captains, give or take. Let's say the company hires 100 pilots before we can get a strike vote, get a cooling off period, and get released. There would certainly be enough Captains to pair with those probationary F/O's but they could only fly a fraction of the company's schedule and not for very long as people started timing out 30/7 and 1/7.
2. Ask the probationary pilots to honor the picket line. You get fired. The union then requires in the T.A. that all probationary pilots who refused to cross the picket line be given their jobs back.
Risks abound to both sides. I don't know which of those two our MEC would pursue, probably based on what Herndon's thoughts these days are...
Hopefully you don't have to find out, and the company comes to a reasonable agreement before a strike happens, but if they don't... they certainly can't say they weren't forewarned.