Hi Pete,
yes i know this can be an issue and already have it on the wishlist. The problem is we are using a Joomla function (JHTML list.users) to get that list and it is not customizable without rewriting it completely into our extension.
There is a small workaround for you -when you inspect the element in your browser (normally right click -> Inspect Element or something like that) you see the html code of the select. And the value is the user id, maybe this is an workaround so long).
Kind regards,
Yves