Upload, no problem, but don't you need download as well for your idea? Texture download is a sticky problem altogether, even with FBO/PBO, and adding it to the API is going to encourage people (particularly beginners) to use it.
Yeah texture downloads shouldn't often be used inside the main game-loop, but I would love to be able to use Horde in my tool-chain as well as in my real-time-renderer.
I haven't used the Horde function getPipelineRenderTargetData
, but it might already give my tools the ability to download the data I want from Horde...