It would be great if someone came up with a multi-harp tuning key, sort of like a Swiss Army knife with multi functions, or one of those screwdrivers that contain different size/style heads inside.
In reverse, when I lost the key to wind my antique grandfather clock, I was able to substitute my harp tuner. So maybe the opposite would work—a clock key could tune a harp. Our local clockmaker has bags full of old clock keys. You could try that and hopefully find one the right size.