Came to think of an alternate solution I used once upon a time (before I got the R2M), a standard softpot ribbon (which is what most of these ribbon controllers are made of anyhow) soldered onto the Korg Monotribe in place of its usual small ribbon. You don't get pressure sensitivity, but the MIDI pitch bend is 14-bit at least so that part works as good as the R2M.
A fun fact, there are also 100cm softpot ribbons manufactured, compared to the standard 50cm!
You can have 8 octaves and still have very smooth and fine pitch control...