Part 1 of the Open Graphics Programming Manual covers the Chrome9 HD Graphics Processor covers the main graphics core and 2D engine. It is 151 pages long and can be found hosted on the X.org server (PDF).
Part 2 is 182 pages and it covers the 3D and video portions of the chipset. It can be found in this PDF.
It's still being analyzed how extensive this documentation is, but on quick glance it appears to covers most of the Chrome 9 registers. Both sets of documents are available under the Creative Commons.
The Chrome 9 hardware isn't exactly new and its open-source support has lagged behind, but due to the lack of skilled manpower working on VIA drivers, the situation may not quickly change (especially when the OpenChrome developers have likely had this documentation already under NDA with VIA Technologies). The VX900 documents complement VIA's previous data dumps for their CX700, VX800, and VX855.