As long as it has the information and _can_ load a kernel with options and _can_ present a UI I don't think the actual UI needs to be standard. I would expect it to be branded by the hardware vendor for example. As long as there is a standard place to put kernel images and a standard way to add/remove labeled boot entries and if you can manually boot custom options using the EFI shell then I think that would solve the problem. I don't think the actual UI needs to be standardized beyond that.