1930s recessed convection radiator cover problem

painter33

As the title indicates I have a house with 1930s recessed convection h/w heating with painted steel Art Deco radiator covers. Just to ID the type of cover, the Deco covers have a center hole for the long stem of the handle that changes the deflection angle of a curved piece of sheet metal that alters the direction and amount of heat coming from the top grilles. Now the problem; it also has a flush-mount bleeder valve self-bolted to the plain face of the cover. Over many decades of use, the stems of many of the valves have been stripped, or worn round (started square for use of a key), and so I can't bleed every radiator. How can I replace the valves? No contemporary valves that I can find attach can directly to the radiator cover, but even installing new valve types means removing the covers that have been painted in place since the late '30s and I would lose direct access to the bleeder. I've tried using a key and needle nose pliers to no avail - the space around the valve stem is too small. What should I do to replace the valves with something available or to install another valve but maintain access to it without having to remove the front cover? Sorry for the length of this.