indeed adding async flag to compile and providing some 'aexec' is a very good idea ! 

*an async repl is really usefull when stuck with a threadless python*

( specific engines, or emscripten cpython )

"top-level async is invalid syntax" : 
Rewinding the readline history stack to get code "async'ified" is probably not the best way : readline is specific to some platforms.
see https://github.com/pmp-p/aioprompt for a hack using that.

First raising an exception "top level code is async" and allowing user to get source code from exception would maybe a nice start to an async repl.