If there's enough room along the shaft of the lifted valve stem (I assume there's a knob on the top),
you could use a small scissors/ratchet clamp, and hold it open that way. They're cheap and available
at hardware stores and home improvement places everywhere. Even Walmart might have them. I
got a pair of them at Canadian Tire. They are sort of like this, only they're plastic, and probably more
likely for woodworking or carpentry applications. They have padded grips.
I agree with Marko, in that it's usually water pressure that keeps it in the "shower" position, at least
in my house shower it is. If I turn off the water, the valve drops and redirects the water to the faucet