It would be if it was possible, but it just isn’t. It’s just down to the way the engine and the new renderer works. It’s a complete rewrite. Besides, we still have UE3 for exactly that reason! I’m personally glad for the single renderer. It made developing in UE3 a bit of a nightmare at times, especially when downscaling a project to DX9, only to find that it couldn’t do a lot of what you were doing in DX11. Mesh-painted reflections was one thing I found I couldn’t work around in DX9.
On the side note, you can pick up a DX11 capable card for less than 3-digits worth of cash now (UK terms over here), which really isn’t that bad at all. The tools are aimed at users with those kinds of computers, there comes a point where its unreasonable to keep supporting less capable systems. I see why its frustrating for a lot of users, but at some point or another the move has to be made.