According to Mick Walkers book 'Japanese Racing Motocycles' in 1985 Honda experimented with a 250cc version of the NR500 oval piston engine fitted with twin turbos, and putting out a claimed 150bhp, they reckoned it would be capable of 175bhp with development, but it never made it past the test bench.
I think the story was that the FIM had hinted at allowing 250 turbo charged engines into the 500 class, so Honda started experimenting, but scrapped the project when the FIM went cold on the idea.
I haven't heard of the sidecar engine you mention, and surely it wouldn't have been eligible for GP's? but maybe someone somewhere managed to sneak the prototype out of HRC's queue for the crusher and continued to play with it?