You could also mount 85/90mm and 105mm lenses onto an SLR -- borrowed or at a camera shop -- to get a sense of the difference between the two focal lengths (this is where sighting on bookcases helps). With some practice, I think you could get used to framing the 105mm inside the 85 or 90mm frameline using some generous mental cropping. I do the opposite with a Nikon S3 -- shooting the 85mm lens using the 105mm frameline -- you get in real tight and know there'll be more space on the edges. If your camera has 85/90 frames, it wouldn't be hard to get a mental picture of where the 105mm falls within those frames. You could also accomplish the same thing by getting the lens's angle of view (this is usually the diagonal, like a TV-screen measurement) and using a protractor to get a sense of its field of view compared to a known finder or lens. I think, in the long run, for telephotos it's better to build a mental picture of the focal length than to use a separate finder. Focusing a telephoto is much more critical than a wide, so it's that much harder to switch back and forth between the camera's main finder and an accessory viewfinder. In a clutch, you can frame a 105 using a 50mm frameline -- the lens, if memory serves correct, takes up about a quarter of the field of view.
It's true that image size on any lens shifts with focal length, but I wouldn't get too hung up on it. Every (or nearly every) interchangeable-lens RF camera in everyday use has to deal with this issue, and they deal with it by ignoring it. Those jobs where precise framing is super-critical are best left to digital or to SLRs with 100-percent viewfinders.
By the way, congrats on that lens. It's one of the best ever. But be warned it's very heavy, more than 1 pound (500+g), because of thick glass and generous brass fittings. On the other hand, that makes it a dream to handhold in low light. I can routinely shoot my 105 at 1/30th of a second and have had acceptable results at 1/15th and, when pressed, 1/8th.
