Just add another radio button in the usercp > options with Show email to guest/ unliscenced members. Then they can make the decision to show email or not then and then below it to show email to unliscenced members or not.
If you want i'll whip up some code for you, well most of it you'll have to do the bit which deals with the checking if there liscenced or not unless you care to explain how :P
$bbuserinfo[liscenced] ??