Christian Konig **is** working on that... without AVIVO code. So the solution you are interested in may/will come WITHOUT any release of AVIVO code.its simple to proof that as wrong because the UVD unit can not handle WebM.
and its simple to proof that wrong again because the HD2900 do not have a UVD unit and the HD2900 will be supported by this solution.
in that way amd try to bring a solution all R300-R500 cards and r600 and all up cards are supported.
1 solution beat them all!
If they do that they do have video codex acceleration support for all hardware generations over the last 10 years on linux.