Hi there,

I have a few questions:

What do you use now?
Do you need the full 18MP of your camera for your presentations or would it be ok to crop? In other words, would the presentation be with static photos or would you "zoom-in" during presentations?
What's the lighting in the room like? Could you do without a flash?
Does it need to be a zoom?
I understand one of your assistants will take the photos, is he/she comfortable with a big camera/lens/flash combination? Does he/she need to take photos from difficult angles(maybe due to space restrictions)? And is he/she purely there for taking photos or with another task as well?

As for using the MR-14 on 77mm lenses. I believe this is possible by using a step-down ring on the lens, 77-72mm for example. Obviously it will block the lens slightly. When using EF lenses on your camera I doubt this will be noticeable. While I was typing this I decided to test it out so here are the results when putting a 77 -> 52 stepdown lens in front of a 82mm lens:

Vignetting Test 52mm on 82mm Lens by Jan Paalman, on Flickr

Vignetting Test 52mm on 82mm Lens by Jan Paalman, on Flickr

The squares indicate the 1.6 crop focal length of your 7D.
I guess one could safely say that you can decrease the lenses opening by quite a bit(30mm in this case) without noticing any changes using an APC camera. The lens used was a Tamron 24-70 on a 5Ds, this lens has an 82mm filter thread. PS: Please don't mind the subject or quality of the images, purely test-material.

As said, I don't know if you must have a zoom. I personally like simplicity and if you only get to use the combination for photography during operations I would go as simple as possible. If you can get away with it the 65mm macro might be a good option in combination with a ring flash.
If you need a shorter focal length then something like a 35mm F2 (IS) might be a nice option in a small package.

I hope my input is of some help to you. Personally I wouldn't mind using the 24-70 F4 either, especially if you'd use it outside the clinic as well.

Good luck,

Jan