I have ended up with some extra Pirastro violin G strings. What would the consequences be of using an aluminum-on-gut G string for D?
It's about the tension. In theory, you could tune a string to any pitch, but going up a fifth would require more than twice the tension.
What problems would increased tension cause? The two main problems are the risk of breaking the string and the risk of slippage at the pegs.
Violin and viola strings are all designed to operate at about the same tension. Empirically, that's a little over 10 pounds, with the exception of violin E strings at about 15 pounds. E strings are already bare wire at that tension; going to a lower tension would require them to be even thinner.
For the empirical data I used, and the math/physics behind it, see this post on another forum.