At this moment, automation doesn't allow to directly query data from a variable with smart values. 

 {{issue.getField({{mystring}})}} 

For example: I have a var

 {{myfieldnamestr}} 

containing a string e.g. "Test", which is extracted from

{{fieldChange}} 

I want to be able to do the equivalent of

 {{issue.{{myfieldnamestr}}}} 

to read the value.