the title says it all. since Users module is still in BWC, it looks like creating a default filter doesn't do the trick.
right now the only available fields that can be filtered in the User selection-list are the following:
last_name, first_name,email,address city, my favorites, my items.
It's been a while since I messed with BWC but you should be able to add your custom fields to
custom/modules/Users/metadata/searchdefs.php
There is also a way to make Users visible in Studio, then you can add you custom fields from there, but I forget how. It involves adding an empty folder somewhere... I forget what or where. Sigh. Short memory!
Jason Eggers was the one who originally told me how... maybe he still remembers
FrancescaS