An update on this capability is now ready for beta-testers who use nodemcu.
The Github ticket is here: https://github.com/nodemcu/nodemcu-firmware/issues/1302#issuecomment-283945689
this includes updates to the documentation with example and the link to the compiled firmware. It has debugging enabled so you will see a bunch of activity on the serial line as you start the process.
The big thing that changed since the last time I mentioned it here is the validation callback which lets you control what to send back to the user (including http status result, content type and body)