Well I may be out of date on this one - I've not asked the MU for an SP recently (I've been a musician these past 10 years, not a TVian) but the last time we tried to do a (bluegrass, actually) folky/sessiony/festivaly thingy the rules where that everyone had to be paid the MU minimum no matter for how long they appeared on screen, and regardless of whether they were members or not. Being a long-standing MUist myself I tried all the obvious angles, but we just couldn't bring it in within the budget, so the show was never made.
They may have relaxed that now. My niece was in one of those fiddle groups - did ToTP with both Robbie and Elton IIRC. They got union rates back then for sure.