I guess it could be done. An udev rule could say identify older legacy gpu's by pci id and automatically download an addon with the drivers. Would't work without an internet connection though and seems like not an ideal approach. Also I think nobody (in ELEC world) cares enough about that old gpus. I'm just speculating though, better go ask in the feature request section.
Guide how to use the older drivers:
1. Follow official instructions how to get the source of LE8
2. Modify packages/x11/driver/xf86-video-nvidia-legacy/package.mk and downgrade the version
2. Scour the internet and find info on what patches are needed for compiling with 4.9 kernel and then put them in packages/x11/driver/xf86-video-nvidia-legacy/patches/
3. Build
Very simplified but that's basically it 