During rehearsal hours only in my experience. The point of the showcase is to leave the actor as free as possible to do other things whole rehearsing for the showcase. You can't make them come in during nonscheduled rehearsal hours.
But like you said, check with equity. I'm almost positive that will be their answer.
If I recall correctly, the answer is actually the opposite of this. (Full disclosure though, this conversation was a few years ago. They could have changed their stance on this or I just just be wrong. But, to make sure I understand their answers completely, I do ask very specific questions, such as "If I'm rehearsing 6-10pm, you're saying I can schedule a costume fitting at 12pm?)
In my numerous conversations with Equity about this, they have told me that working on a Showcase is basically the equivalent of volunteering one's time, and so fittings can be scheduled at any time during the day, as long as the actor is amenable to that time, and as long as the fitting is included in the total number of hours allowed per rehearsal day, which on a regular showcase is 6 hours. You'll notice that on the Showcase code there is no span of day limitation, so it's 6 rehearsal hours, not 6 out of 7. Theoretically, a fitting could be scheduled for 11am, and rehearsal scheduled for 7-11pm.
In my past experiences on showcases, I personally always try to schedule fittings during (or immediately before or after) rehearsal unless it just isn't possible for whatever reason, and then move on to hours outside of rehearsal that are at the actor's convenience. A very specific example of this would be fittings at the Costume Collection, where fittings on-site must be scheduled in advance and only during their specific hours of operation (9a-5:30pm and last checkout at 4pm). If you're working on a show that only rehearses in the evenings, scheduling a fitting during rehearsal hours won't be possible.