The reason you don't see harness bars is they are a compromise. If you are going fast enough to need a racing harness, then you also need a roll bar or roll cage. It sounds like maybe one of these products could serve your purpose. They fit into a finished interior and afford you a higher level of protection. Mounting the shoulder harness is critical, so read up and don't put yourself in a dangerous situation. Generally speaking, the lap belt should hold you down into the seat, the shoulder harness is to hold you back against the seat. If the shoulder harness is pulling you down into the seat, that's bad. And if the horizontal bar the shoulder belts are attached to is too low, that's what happens, the shoulder harnesses will compress your back like Bonzo and the others have stated.
There are probably other manufacturers, but these guys have a pretty good reputation in the autox arena. I wouldn't recommend a bolt in bar for anything more than autox or the occasional HPDE track day, beyond that you should consider a custom built bar or cage.
Maybe the street sport bar is a workable solution?
http://www.autopowerindustries.com/applicationlist.asp
Chris