get SHA1 hash

I was hoping that a require("crypto") would look for a native install first, and then look in modules on espruino site.

Yes - it should do this, but it needs a bit of tweaking on my side first as currently the list of internal modules reported by espruino.com isn't entirely accurate.

and then attempted to add that to the modules cache

You could probably do something really hacky by modifying the module cache directly:

global["\xFF"].modules["crypto"] =  global["\xFF"].modules["http://..."];