It is possible, very easily, and I use "reverse" status bars in my oUF layout.
Basically instead of:
Code:
statusbar:SetValue(value)
You just do:
Code:
statusbar:SetValue(maxValue - value)
Note that for oUF this means you will need to run your own health update function, either OverrideUpdateHealth or PostUpdateHealth. It's also slightly buggy (for oUF) in that the first time you see a frame (other than the player unit) the bar will probably be "backwards". It will fix itself once the health updates though. I should probably report that to Haste...