Hmm...I'm not sure if I'm reading your question correctly (and the fact that my site is temporarily down so I can't test it) but try wrapping a conditional around the content you want hidden only to those who did not fill in the custom field such as:
Code:
<if condition="$show[fieldx]">
Memberlist content
</if>
Basically, if I did this correctly, if a user had filled in "fieldx" then user will show up in Memberlist.