For example, could the Nouveau driver be used and provide hardware accelerated drawing when Android is compiled for x86?
Noveau is a driver for the X.org graphics system so it definitely couldn't be used verbatim. The one part that I'd expect to possibly be useful would be the code that actually talks to the hardware. The same is true for any other X.org driver.
But in Noveau's case, most of the work is in reverse-engineering the hardware interface as Nvidia refuses to release the documentation. This work could definitely be reused in a hypothetical Android Nvidia driver.
Almost none of the regular Linux graphics architecture is used on Android (that is, Mesa, XWindows, etc). Driver developers can choose to reuse major portions of their drivers, but there is a lot of plumbing involved to get drivers to work with SurfaceFlinger (and gralloc, and the Hardware Composer), including 3D rendering as well as display output (HDMI support, etc.).