I'm curious if it is possible to add this vas usage option at runtime.
This has been already discussed and answered here:
http://www.fsdreamteam.com/forum/index.php/topic,7960.msg88500.html#msg88500It must be done at install time to preserve the maximum texture quality, because they are not just resized, but regenerated in the correct resolution starting from their original sources, which is better than keep resizing them up/down, and it's better than storing all the different size variation of the textures, so none of your hard drive space is wasted for a resolution you don't use at this time.
Note that, if you install 4096 textures, but set the texture resolution to 2048 or 1024, you WILL save memory, but the textures might load a bit slower (because the file *IS* larger), you will waste some hard drive space, and the visual result won't be as good as if the texture was regenerated from scratch in the desired resolution, which is what the installer does.
Also, the installer gives you the flexibility to choose the resolution of the buildings, the ground polygons and the dynamic shadows separately, compared to the HD texture resolution option in the Addon Manager, which will set *everything* at 1024, 2048 or 4096.