Interesting discussion. The idea that an additional element in the optical train can improve color correction in a telescope is totally reasonable - all that is required is the element compensates for errors introduced by the objective lens.

Most of us think of the compensating element being part of the objective lens, like the flint glass in an achromatic lens, but there are many other ways to reduce chromatic aberration along the optical train. Schupmann and petzval designs are examples that use medial elements to improve "color correction" in telescopes.

-Stephen