Posted in the hope that this saves someone else’s time. Examples in the documentation would be goood… possibly this may have been mentioned before on the forum. 
$rx.ext.call
This method allows you to call an extension.
Look up the correct parameters for context and name in the extension itself under the System Design tab in the Workbench.
Basic usage (e.g. in a binding definition):
$rx.ext.call(“global/percussion/generic/”,“sys_ToUpperCase”,“text string”)
With a field value as a parameter:
$rx.ext.call(“global/percussion/generic/”,“sys_ToUpperCase”,$sys.item.getProperty(“rx:sys_title”).String)