I work mainly in SPT, and if I was in your situation, I’d politely tell the designer that a 10am fitting just is not possible given AEA rules. I would probably also get the Production Manager in the loop, to see if they can help with scheduling the designer to be in town at more useful times. The designers I’ve worked in SPT settings have always expected to work around our rehearsal schedule; is it possible the deisgner in your case also knows this, and just figured that asking you to bend the rules for them was worth a shot?
a number of our actors also work administratively, so they will sneak into a 2PM fitting.
If an actor initiates something like this, I don’t know that I’d oppose it, even though it’s not by the book. But I do not think it’s appropriate for the designer, or an SM, to ask that of the actors. It puts the actors in a bad position; they may worry that they can’t say no because it will hurt their relationship with the designer (and they’ll therefore end up with a costume they’re unhappy with). To me there is also a big difference between a 10am fitting and 2pm fitting when the rehearsal isn’t until 6pm. 2pm you could possibly justify if you released the actor at 9pm (2 hour fitting consecutive to 5 hour rehearsal call as per SPT Rule 44-L (even if in reality they get a 1/2 Hour fitting, then a break until 6pm when real rehearsal starts).
Matthew is onto something with having fittings during rehearsal hours. You can do this even within your 5 hour block. For example, if John Doe isn’t in the first scene, call him at 6pm anyway, but have him do a fitting from 6-6:30, then join the rehearsal when you move on to scene 2. Or plan to let the actors out for 15/30 minute fittings at various points during the regular rehearsal hours when they won’t be needed in the scene. Just make sure the director knows the actors who’ve had fittings still need to be released at 11pm (they can’t keep anyone later to make up time they were out of the room).
I’ve even done this with a two person play where both actors were onstage the entire show; each character had a long speeches or detailed moment that the director could happily work on with just one person for half an hour while the other actor was out of the room.