Safe URI - Can we have anything other than https, http, ftp and mailTo

1. No - we don't have the option to expand the list - the documentation also does not mention any configuration at server side that can be done to expand this list. docs.appian.com/.../Appian_Data_Types.html
2. The above ones are not allowed when creating safeURI by means of casting from text.
3. Try one thing: check if urlwithparameters function works for you: docs.appian.com/.../fnc_scripting_urlwithparameters.html

I am skeptical it would work.
But - no harm in trying.
Pass the last two parameters as blank arrays. And in the path use the complete URL.

I think however - it wont work because Appian may require that the path be some path internal to Appian application. Let me know if it works.