I am trying to create a query string of variable assignments separated by the & symbol (ex: \"var1=x&var2=y&...\"). I plan to pass this
Using the disable-output-escaping attribute (a boolean) is probably the easiest way of accomplishing this. Notice that you can use this not only on but also with , which might be cleaner, depending on your specific case.
Here's the relevant part of the specification: http://www.w3.org/TR/xslt#disable-output-escaping