I do not think dynamic DEX generation and loading was ever possible with Dalvik. So ART makes this no more or less difficult. An Android runtime environment, by definition, includes all the code to interpret DEX, even if it isn't made to be capable of using dynamically generated DEX.

But you have other problems to solve: How are you going to generate DEX? Generate java bytecode and bring dx to the runtime? Make jruby emit DEX directly?

EDIT: As per the comment below, that is how it's done. You generate Java bytecode and run dx on it.