Some people do it by holding in the swivel button and simply forcing the flash further to the right - presumably breaking off a little plastic tab that is designed to prevent it from turning farther than 90 degrees. That apparently will "allow" it to swivel to 135 degrees - and probably put an unacceptable strain on delicate internal wires - which I presume is why Nikon limited the swivel in the first place.
The question is why? You can always mount the flash on a bracket or stand with a swivel mount that will give you 360 degrees of rotation without risking damaging a pretty expensive and difficult to replace flash.
Chris and George: thanks for the replies. The reason for wanting the ability to swivel equally in both directions is that I prefer to keep the flash on camera in most situations. In this manner, I can control the direction of the light while not having to use light stands, cords etc and can move around freely. Nikon's latest speedlight models allow the ability to swivel 180 in either direction which is marketed as an improvement. I did buy a 900, but it is so prone to overheating even with the thermal cutoff feature turned off that I no longer use it. The question remains is can the 800 be modified (not forced) to rotate farther? Very likely the answer is no, but perhaps there is a solution. Thanks again, Richard