Hey
I've got an idea that might be worth trying, it's a bit complicated so I'll try to explain as simply as I can.
You could create the entire 3D world in DB but keep it separated from the characters on screen.
Then you could use a second camera to draw the background onto a bitmap setup the same size as the screen and grab that as an image.
Then hide the entire map so DB ignores it and use the grabbed image like a backdrop beneath the 3D characters.
I'd recommend using 'draw to back' and disable the backdrop for the main camera, then just paste the image onto the screen every frame.
Doing this will probably make the game run like lightning, since it'll only draw the background once each time the camera moves. After that it only needs to draw the characters and whatever special effects you use.
A few words of warning, this trick will have absolutely no zdepth interacting between the background and the characters, so careful positioning of the cameras and scenery would be needed so that no scenery appears on top of the characters when it should be behind them.
You could try to use multiple cameras with different draw ranges to produce separate 'layers' of scenery and put the 3d characters inbetween, but how you'd get images pasted both behind and infront of the 3D would be quite a challenge.
Avan
The glass is half empty...
That way there's more room for improvement!