Here I thought that my two 4000-series systems (4850 and 4890), which do not reduce memory frequency in either 2D or UVD mode, were truly behind the times. It appears even 6000-series cards don't do it while in UVD mode. That alone is something which could be enabled by AMD and it's board partners on all new cards by default. But, seeing as the default memory speed provides the best benchmark numbers, that's likely the reason why it's not done.
I typically use CCC's Overdrive to manually reduce memory speed while in 2D-mode, which works fine. Despite having been manually set, whenever it senses UVD playback (be it Flash, DivX, etc.) the memory frequency changes to it's default speed (1000MHz on the 4850, 975 on the 4890). Now I know why - once UVD mode is active, it's clocks reign supreme. It seems the only way to change that is to mod the BIOS.
When it comes to general users, the end result of all this tweaking seems to unfortunately only save rather minuscule amounts of power.