from what i gathered, the th8 rs works independently from any wheel, including the t500 rs. It doesn't rely on communicating with a wheel in order to work. You have to plug it into its own usb port, even on the ps3. A game then has to be programmed to support it separately, since it can't be tethered to the wheel. As a consequence, this would enable you to use it with any wheel on the ps3 as well, unless thrustmaster deliberately requests that game developers lock out use of the shifter unless the t500 rs is detected.
Remember how gt5 supports using a control pad alongside using your wheel? My guess is that it wasn't added in there by coincidence. The design of the t500, which has no connection port for the shifter, supports this thought as well. They always intended that the th8 rs would be it's own separate product, even though its a part of the rs line. This makes sense as they can make more money marketing it this way.
The ultimate downfall here is that th8 rs owners who race on ps3 only, are at the mercy of game developers. Developers must go out of their way to program support for the shifter into their game, similar to how gt5 was programmed to support the shifter.
People need to make sure they get on game developers' asses to support this shifter on the ps3 for any up and coming race titles, or they probably wont do it due to the amount of extra work it would take.