We did give this a try and ultimately ended to not do this as it causes a number of consistency issues. It was a while ago and my memory on it is a bit fuzzy since I wasn't working directly on it but iirc what is came down to was the resource bar is fundamentally meant to function as a resource bar, doubling up on the buff bar goes against a pretty solid rule of thumb in UI/EX design which is limit special cases in UI and avoid duplication. Breaking consistency here has a huge long term cost that can be difficult to quantify.
From a gameplay perspective, the resource bar is meant to be a relatively stable and slowish moving bar showing a characters relative strength, current threat level, and reveal their general flow of attrition (how long it takes to be exhausted). Seeing a champion low on energy or mana means they have limited casts available to them and is a moment of weakness enemies can exploit. In this case for the opponent this bar would be incredibly misleading with Riven being most dangerous but also most vulnerable when this bar is empty. With some weird middle ground of when partially full you're most likely being killed.
The second important gameplay element is that there should have some level of consistency and flow in the resource bar. Rarely do mana or energy users dump their entire bar in one go, while Riven can cause the bar to fill and empty in only a second or two. It can be incredibly tempting to see an open spot on a UI and want to fill it (believe me I've been there, as a mechanics designer I want to cram as much information as I can pretty much everywhere) but there is a steep clarity and readability cost which increases exponentially each time you break. Sometimes this cost can totally be worth it, in this specific case we didn't feel it was. I would love to explore the space of custom UI elements only the player controlling the character can see/interact with though, and this seems like a logical candidate for that.
