More: BMW C 650 GT Blog
Here's a quick look at the adjustable backrest on the C 650 GT. It has three bolts underneath and three adjustment points. The adjustment is made but changing the bolts underneath the seat.
The backrest moves forward about 50 mm total, so there isn't much adjustment. I'm 5'10" (183 cm) and I could use more adjustment towards the rear, not forward. The backrest does not move up and down.
I'd guess that if the C-series scooters become popular, someone will offer seat options and perhaps different backrests. It would be fairly simple to develop a backrest that could bolt into the existing holes.
Publication Date: November 2012
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From "S.D." (May 2013): "I have the scooter. Replace the bolts with longer bolts and then place steel tube spacers between the seat and the bottom of the backrest and tighten the bolts. Can be raised by as much as 3 to 4 inches without affecting the stability of the backrest.
I painted the spacers black so as to look factory. Functions extremely well and can be removed without any damage to the seat or the backrest. Provides exceptional lower back support."