The driver needs to support 3 basic things:
Composite + Render + ARGB GLX buffers
The Orbiter code is written entirely using X.org standard extensions. My understanding is that the VIA chipsets, at least the latest ones, can support full alpha blended overlay. The problem is that #1 VIA doesn't release the specs, except under NDA & #2 there aren't a huge number of developers who have the skills needed to extend the OpenChrome drivers to support OpenGL + Composite in the drivers, especially for free.