Hey Joe thanks for alerting me to this. I see your point about needing dynamic access too.
Let me think about it. When we did dynamic it was kind of a hurry and I knew there should be a general mechanism. My first thought it something like what shells does, $(code).
It might as well work for all magics?

Yeah, the possibility of having a generalized substitution mechanism crossed my mind, too. Doing $(code) substitution on all magics doesn't sound crazy. It might be weird if that code had any side effects, but that would probably be covered by a general "don't be surprised if doing strange things gives you strange results" disclaimer. If the code threw an exception, you'd want to report it sensibly. But if that all happens before the final version with substitutions is processed, the existing magic commands might not need any changes.