I’m slightly confused. Ultra HDR is only a file format adjustment. Specifically, it’s a bit of extra metadata in the exif file that encodes a gainmap which is calculated from the HDR original, and a tonemapped version of said image. The latter then becomes the base image.
It barely supports editing existing SDR artwork;
Its painting logic is counterintuitive for general creators;
Neither of these would be fixed by ultraHDR support.
I am actually working on HDR handling right as we speak, and while I’ll be doing a tonemapping filter, I don’t have plans to handle gainmaps themselves just yet, as I right now have too little information to implement it.