We discussed this several months ago. I can't remember what we came up with. We do know that the G20 is based off the Primera. Obviously, the cars are based on two different platforms. However, Nissan seems to use "parts bin" technology in their cars to save costs. It is possible that the seats would have the same bracket. Even if it didn't, you might be able to graft the G20 seat onto the Sentra mounting bracket.
Have you priced out any seats from a salvage yard?