Tapping into the external tankless water heater for outdoor shower
I would say it is fine. ( code issues aside )

If you do not want to drill new holes in the side of your house then I would suggest to not use your hose bib. 

If you are going to be plumbing a new line anyway you might as well plumb the cold as well and save your hose bib for alternative uses. Presumably you have an accessible cold water supply to the water heater and plenty of water pressure, T off the cold going into the WH for the cold and T into the hot coming out of the WH. Run both lines ( copper or pex ) to a shower mixing valve, put 1/4 turn valves in line so you can service/shut it off.

I would build some sort of cover/housing to put over the supply lines to the mixing valve to protect them and for aesthetics.  

As to freezing. Keep in mind the planet will (more then likely) experience another ice age. South Carolina will freeze/thaw many time before a full ice age ensues but humans will probably be long gone. ( Either through self destruction or leaving this planet to find another planet to  ---abuse for profit?  . )  

The planet will survive long after any parasites render their host uninhabitable.  ( alter its climate to the point of not being able to support the life that has evolved to exist within its specific climate parameters. )

But I digress.  

I think you should ( PROBABLY ) be good for the near future.