I’ve played with this a bit in the meantime too. (Your question was useful since it made me think about this a bit more.) Here’s how SnoopCompile now deals with this:
ft2f (“function type to function”) is only used to print stuff to the REPL, so all that was needed was a good fallback for the case of a closure. And yes, just as you say I do have to Base.unwrap_unionall it before calling ft2f.