I didn't help much at all .. just brainstormed a bit out loud to hopefully point you in the right direction.
Unfortunately .. as mentioned by Boothby in the other thread at vB.com .. you still have the issue of things like search not working.
You wouldn't have to worry about people clicking on a PM button to sent members a private message ... but users would get errors if they fill in the field manually. Even worse .. Case where User1 has display name Frank, while another user has Frank as his real username. The wrong member would get the PM. Same thing with manual searching unless they used the searchlink directly from the person's post or profile.
For the who's online page .. I'm only guessing here and haven't tried it myself .. but in the "whosonlinebit" template, change this:
Code:
<a href="member.php?$session[sessionurl]u=$userinfo[userid]">
$userinfo[musername]</a>$userinfo[hidden]
<if condition="$show['buddy']">+</if>
into this:
Code:
<a href="member.php?$session[sessionurl]u=$userinfo[userid]">
$userinfo[fieldX]</a>$userinfo[hidden]
<if condition="$show['buddy']">+</if>