Write/Create/Manipulate User Variables on Player with Javascript

My current process for writing to user variables from an attached HTML/JS is very cumbersome:

1. Create the variable in presentation properties
2. Modify .brs (htmlZoneMessage) attached to presentation
3. Use bsn MessagePort in HTML file to write to variable

Is there an easier way to do this without changing the .brs every time I need a new variable? 

Note: using BA 4.7.2.18, not BrightAuthor:Connected