[Kirk Y]

Create a new template containing what you'd like displayed when your variable is used.

Create a hook using location "global_start" with the following:

eval('$myvar = "' . fetch_template('mytemplate') . '";');

Replace "myvar" and "mytemplate" accordingly.

Anywhere you use $myvar in a template, the contents of mytemplate will be executed.

To cache the template, create a new plugin at hook location "cache_templates" with the following:

$globaltemplates[]  = 'mytemplate';

Again, replacing "mytemplate" with your template name.