I just confirmed this. From LOA 67 Informational Picket Email dated 03/22/17:

We are very appreciative of the number of fellow pilots at other carriers who have asked to join us on the picket line. This is a great gesture and it shows the solidarity all ALPA members have for each other, and we are also grateful for the Frontier flight attendants who have asked to walk with us. However, at least for this first time out, we would like our picket to be a FRONTIER pilot-only show. We want there to be no mistake that every pilot there is a Frontier Pilot, as this is the clearest way that we can communicate our unity, and because LOA 67 was a very personal and unique sacrifice made by our group alone. As we plan for further events in support of our ongoing negotiations, we will welcome pilots from the many ALPA carriers who want to join us.

Many of our probationary pilots have also asked how they can support our picket, and we are grateful for that support. But because you are still considered apprentice ALPA members and do not yet enjoy the protections of full union membership, we urge you to not walk the line on April 19. Instead, you can support your fellow pilots by keeping your reserve days, or if you're a line holder, consider picking up open time so that someone off probation can come to the event and walk. By helping cover the operation so that those off probation can picket, you will be doing your part to support those of us walking the line.

I added the CAPS merely for emphasis on the important sentence! Thanks for the support ovrtake92, and everyone else!