I had the same question and ended up buying the CV 35 mm viewfinder today to use with a Zeiss 25mm on the R-D1s. I went back and forth between it and the 25D (which I think is identical to the 40mm CV finder). I decided either would probably work to frame with, but the 35 was much better built (metal instead of plastic, identical to the CV 28mm finder), somewhat smaller, and much nicer in general.
An evening of shooting has lead me to conclude the framelines will work for me for the 25mm. Not an exact match, but close enough. Enough to make me wonder if the 40 wouldn't be too tight...
The only catch is that I paid more for the 35mm finder than I would have for the 25D or the 40....
I wondered about the hassle of using the rangefinder for focusing and the additional viewfinder for framing, but you get used to it pretty fast, and I actually prefer looking through the external viewfinders. They are brighter and easier to see the framelines with if you wear glasses, as I do. It is also pushing me to try more range and hyperfocal focusing, which is not a bad thing to learn.
(I should have mentioned that I have been using the CV 28mm finder with the Color-Skopar 21/4 on the R-D1s for a couple of months and it is a near-perfect match.)