The Canon adaptor is pretty new and rare so it's hard to find anyone with much experience with it. I saw Vince Pace adapting some for his F950 rigs.
It makes a long camera even longer, which can be a problem. Otherwise, assuming you can live with the extra length and having to flip the image, it should work fine. You may have to live with a squeezed-looking 16x9 image on the set monitor and in editing until you eventually make a 2.35 letterboxed version at the end of post for home video.
Most people would just frame 16x9 HD for cropping in post to 2.35. I've shot five HD features that, but using the 1080P Sony F900, not the 720P Varicam. The Sony F900 allows you to create framelines in the viewfinder for that (Vista 2 is a preset version for that in the Marker Menu) and I seem to recall a similar ability to set scope framelines in the Varicam.
I'm hestitant to crop a 720P image to 2.35 for a final transfer to 35mm. Even with 1080P, you're cropping to about 800 lines to get a 2.35 image.
