If you look precisely into the focusing screen chamber access of the 7D and the 5D3 there is a difference.
I hope the 5D3 will still have the same ease of access. Actually, they can't change it a lot, can they? Servicing / cleaning of the focusing screen is performed quite often.
Well, I must say: it's easy to access. I can even confirm it, but without illustrating the whole procedure on my own body, because I'm a bit afraid to do it myself.
First of all: DON'T YOU EVER TOUCH YOUR FOCUSING SCREEN WITH ANYTHING. USE ONLY AIR BLOWER TO REMOVE VISIBLE DUST ON IT.
As for my experience: don't even use a blower, because you may make things worse, as I did it with my copy of 5D Mk III.
A little story here:
About a week ago I noticed some speck on my focusing screen while looking through the viewfinder. Firstly I didn't plan to do anything with that. But whom I'm fooling around?!
It was annoying every time I looked through the viewfinder. Well, I decided to try a blower (damn it).
2 pushes on a blower, then another 2 and another and finally it (the speck) was gone. I was happy for 2 seconds or more before I noticed new participles from the other side of the focusing screen. I tried to blow them away but nothing helped. It was under the focusing screen.
The things got worse when I tried a sensor cleaning swab. My bad. It didn't helped either. As a service guy said I had scratched my focusing screen.
As you've already know I went to service. The screen was scratched but not noticable in the viewfinder. I asked them to try to clean it from visible participles. As they said it was easier to replace than clean it.
But as it's a newer model, the new focusing screen will take for about 2 month to get shipped.
I was begging them to clean it, just to try, and the service guy said OK.
I must mention that this was not an authorized canons service centre. In my country (Ukraine), the service under warranty is not so good and professional as the paid one, with "real" engineers working there.
The service guy took my MKIII and invited me to come with him just to watch the whole process (madness). He unscrewed two little black screws (as on the pic) and pulled out the bracket, which holds the focusing screen chamber.
The chamber opens by clicking the lock, as on the 7D's focusing screen replacement procedure (http://www.focusingscreen.com/work/7den.htm
), and then the screen was exposed. The service guy took the focusing screen out with tweezers and pushed some air on it. It was cleaner than before, with less dust but some specks were "glued" and didn't want to fall of.
So we ended up with ordering the new focusing screen. And now I'm waiting it to get shipped in two months.
That's the whole story. Sorry for such a long one.....