Found a hydrostatic motorcycle. No mention of an accumulator so not technically a hybrid, and the mpg isn't great, but it isn't too horrible either (~65mpg average).
Top speed 75 mph, engine rated at 24.8HP.
For comparison, Suzuki GZ250 top speed is 76mph, 20hp, rated @ 82 mpg
So the hydrostatic lost 1/5 of its horsepower somewhere, and 1/5 of its efficiency. I don't know that the bikes have significantly different CDAs with an upright rider on them, it is all pretty blunt. Plus the diesel seems to have lost any advantage of being a diesel.
Hydrostatic Drive Diesel Motorcycle
Of course recapturing braking would make a lot of sense, but this is just trying to compare apples to apples and break down to the detailed efficiencies. You can add a hydraulic motor-pump and accumulator in parallel to a "regular" bike too, and on the surface it would would yield even better results than forcing all the engine power through the hydrostatic drive.