OK, so after 4 burnt regulator-rectifiers I felt that might go down to the battery cables (or their connection). Turns out power wire (+) that goes from the battery to the starter relay was superthin, bent and of lousy quality. Ground wire (-) that goes from the batter to the engine/frame was connected on the left side of the engine, close to the ground, near to the front sprocket. In the dirtiest place on the bike, that is. I relocated the cable and mounted it on the frame, at the place where the engine mount is.
BTW, when I did that, I checked all the engine mount bolts and they were loose. I somehow expected that and tightened them up. I guess they went loose because of the engine vibration, but I'm not sure. Could've been like that right from the factory.
The new cables that I installed are actually car grounding cables from an HKS set. Cost me some 30 yuan on taobao.