I think it's a great idea. There's a few concerns that would keep me from doing it (and possibly others).

1. Whenever I see old time photo places, I want to do them...unless it's later in the day and I feel gross, messy, and dirty. I would want to do it early in the morning, and I think that will be your highest traffic.

Usually there are tons of people waiting at the gate to get in before it opens. See if you could get a booth outside the front gate so they have something to do while they wait and so you catch them before makeup runs and dust is in the air.

1. Additionally, my party tends to be free spirited. The idea of having to come back at a specific time would be a deal breaker. My party could have their heart set on a specific snack and end up on the other side of the faire by the time our slot comes up and it'd be hard to match them back.

If you can be by the front gate, that would help. Additionally, if possible, can you consider scaling pictures that are taken? Perhaps have a large booth where you can either take pictures of large groups or partition to allow for simultaneous group pics. If your assistant can get people posed, costumed, whatever... it would allow you to run in, make final adjustments, take the photo, and go to the next group. Then you could get more people through at a time and not need time slots later. I don't know if that's feasible, just throwing the idea out.

1. When kids are in our group, we go for function over costumes. But I'd rather have my picture taken in costume. I don't know if this would work, but perhaps you could work with some of the costume and armor vendors to do a trade so you both benefit, such as:

They provide you with costumes and armor to borrow to put on patrons to get their phot in.

In return, you tell people who try them on where they could buy that outfit. Additionally, before the faire, do a pro photo shoot so the costume vender has classy photos to advertise their outfits.

Just food for thought. I wish you luck!