We were able to do a good deal of the work from the inside, especially since the hatch didn't have a proper lid. After unscrewing the sheet of plexi that was put over the opening, all the rivet drilling could be done from the inside. If it did have the lid/screen pieces, you could do 3 sides of the vent from the inside. The new rivet hole drilling was done from the outside, with me pushing up from the inside to help hold everything in place. Then the new rivets were put in from the outside also, so there was less movement of the vent assembly and better leverage angle on the rivet gun. Overall, i'd say about 40% of the process was done from the inside, in our case.
Also, with using ladders, there wasn't much laying on the roof required, but that could have to do with the fact that Ollie is about 6'4". There was definitely more leaning/sprawling required for me, at 5'6".