I think the answer to that question is probably not… I no longer have a running 5.7 system, so it’s not something I deal with regularly.
On the original question, I believe that the rxGlobal templates are just that: Global. There are ways to include variables in your global template that don’t involve using the rxGlobal templates.
Look at the templates that included in your global XSL, many of these templates have names that start with “rx” and other with “sys”. You can modify the “rx” templates and put conditionals in them. This won’t be simple, it probably is possible.