Mrs. Trotsky recently acquired a delightful 1987 Schwinn Super Sport with 6 speed 600 SIS shifting. She has some back problems that limit mobility, so downtube shifters are an issue. Can I cannibalize any old pair of stem shifters to move the levers up, or will that not work for some reason?

Yes, I'm aware I could do bar ends, and yes, I'm aware IRD makes a clamp-on mount for this, but $50 seems like a lot for a stem clamp if I can get one for $5 at the co-op.