Sounds awesome. Makes me want to dust off my D3 installation media so I can try that out.
I see no reason why you couldn't use the BINK video support in FFMPEG to add video support to the open source version of Doom 3 BFG...
So why dont you add it then
Max's post nicely sums it up. IdTech4 is practically ancient technology at this point. You're not going to see many games using it until the renderer is modernized and optimized to be even remotely competitive with the other idtech based engines that have evolved beyond it.