I'd like to be able to parameterize the value of the
external-destination. Can it be done?

Get your favorite source of XSLT facts and check it for
the term "attribute value template", or AVT for short.

It means that for attributes which aren't evaluated
as XPath by default, you can use braces to denote
XPath expressions:
 <fo:basic-link
   external-destination="{$parameter}">...</fo:basic-link>