Hi David,
Thank you for offering to write this - I think that this is a great idea! I would, however, make sure to co-ordinate with [PROPOSAL] Rename `flang-new` to `flang`. That proposal seems to be gaining traction, so things are indeed very promising!
I am happy to either review or even contribute myself. From my perspective, as a “Flang driver” code-owner, I’d like to call out successful adoption of clangDriver (Clang’s compiler driver lib) to implement another upstream compiler driver 
Thanks!
-Andrzej