only way to do it is to tell the cast straight-up it's a non-union shoot and give them a contract of your own to sign if they are willing to break from their union to work with you.
As much as a pain in the a*s, it's worth it (we did the ULBA for my film and of course, I'm SAG as well.... but I would have easily broke from the union to be in my own movie but I would have never asked any of the other actors to do so).
There are some states that have a 'right to work' clause where the union won't reprimand the actors (not sure where you're from). In all fairness, the actors worked their way into the union and they should stay there. If these actors are worth your project, do the agreement. If your shooting schedule is on the line, SAG may even be able to get things moving along quickly. It may seem daunting, but the end result is worth it. Trust me.
Just thought I would drop in and post my SAG experience:
Stricken www.strickenthemovie.com was SAG Ultra Low Budget. I had two SAG actors and the cost for employing them came out (After workman's comp, etc. to 150 an 8 hour day. But the minimum is about 100 which equals 150 a day.
The process was painless as I went through Talent Fund to handle my payroll. They do not take a big cut and are very worth it as they handle all the taxes etc and I never worry about it again. I just fax them an exhibit G after each filming day for each actor.
I worked with a woman named Fatna in the San Fran office. WORK WITH HER IF YOU CAn and say Matt Sconce from Stricken recommended her so she immediately feels like she knows you. She was incredibly transparent, flexible, and helpful, walking me through each step of the way, and following up and helping me when I dropped the ball. David Fine also helped me understand the process. All in all, things went quite well and, to get David, it was worth it.