You may be able to find a used 28mm finder of some sort. Maybe one of the turret finders has a 28mm frame. The low-cost route would be a home-made 78°-diagonal viewfinder... But this would give you a view without parallax correction. The Bessa-R series might approximate 28mm in the full area of the viewfinder, I don't know, but you wouldn't have parralax correction then either. I have tried this approach with two different cameras... it works, but demands more of the user.
My Bronica RF645 has framelines for 100mm and 65mm, and there's an external finder for the 45mm (28mm equivalent) lens. Which I have never used, as the full viewfinder window does approximate its field of view. Without parallax correction of course. But then the external viewfinder has no correction either except an additional close-focus cut-off line, and even more parallax error than the camera viewfinder. So I unexpected cut off bits and pieces in the close-in pics and try to learn to move the camera slightly before pressing the shutter to compensate for the error. Frankly, it's easier to use the Fuji 645 instead, with its fixed 45mm lens and viewfinder with both parallax and framesize corrections!
I also attempted this with the Minolta CLE and a 25mm CV lens, as its viewfinder approximates the field of the 25mm lens around the outside of the 28mm framelines. Same deal about lopping of bits of my close subjects, which is most of them, and I switched to a 28mm lens instead. Ah, now I have parallax correction!
So, the upshot for me is that what you propose does work, but it makes you work harder too keeping in mind the need for manual parallax correction. If your subjects are mostly more distant ones, then no big deal at all. If you're serious about 28mm in tight like I am with environmental portraits, then I'd seek out a camera with built-in parallax corrected framelines for that lens. Maybe a Hexar RF or Minolta CLE, a used Leica M6 with .58x viewfinder, maybe save your pennies for this Spring's intro of the Zeiss-Ikon RF camera.