With limited right-knee flexion, can a DAS-legal motorbike have the footpegs, rear brake, and gear lever adapted to reduce knee bend—and which bikes would you recomend?

If you are considering the option of adaptation to forward foot pegs and forward controls on a motorcycle with a manual transmission, most sports and/or street models are not really suited to that type of adaptation.
Cruiser models tend to be by far the best suited to this type of adaptation and the riding position also lends itself better to a feet-forward posture.
I recommend you look at these Category A DAS compliant cruiser models:
• The Honda CMX1100, (which is also available with the excellent Honda dual clutch ‘DCT’ semi-automatic transmission variant).
• The Kawasaki Vulcan 650.
• The BMW R18.
All of these models have a relaxed riding position as standard, and they have a length and rake that can easily accommodate forward position foot pegs and controls.
Some Bobber style bikes may also allow a reasonable amount of forward foot peg adaptation, such as:
• The Triumph Bonneville Bobber.
• The Royal Enfield Shotgun 650.
• The Moto Guzzi V9.
• BMW R12.
• Harley Davidson 833 Sportster, which is a cruiser/bobber.
Try going round a few dealerships and sitting on some of these cruisers and/or bobbers to see which nearly suit you best, then the chances are, the forward foot peg and control adaptation should complete the job for you.