I think best of all it looks clean.
For me it's a bit different as the bars I've been messing with go right to red after you've used it (and obviously stay red as I haven't messed with them lately).
I like your border/no border idea. I think the borders work well if you're gonna have a bar showing all the time (like green is ready, red is refilling). But for the ones without borders, I like your color scheme better. Though with your color scheme I would think you would fill the bar left to right (go from red to green), LOL. I'm babbling.
Feel free to post your code. If anything I just like to see if to spawn new ideas for other windows, and the such. There seems like so much potential for new mods, but it just requires some ideas first
Did you figure out how to make it works for hotbars that are not horizontal only (I can kinda care less if it's not possible, but I'm curious) ?