First, I didn’t flush the rewrite rules so, or course, the rule wasn’t being included.
Second, I ended up going with a MUCH simpler solution to passing the variable to any page with the template applied, which was add_rewrite_endpoint
.
add_action('init', 'my_add_endpoints');
function my_add_endpoints()
{
add_rewrite_endpoint('myendpoint', EP_PAGES);
}
!IMPORTANT – add_rewrite_endpoint
registers a query var for you and sets that var’s value to whatever directly follows it.
So this url:
http://mysite.com/mypage/myendpoint/myvalue
becomes:
http://mysite.com/index.php?pagename=mypage&myendpoint=myvalue
It took me two days to eventually figure out what should have already been written in the codex. I hope this helps whomever comes across it.