great plugin! i have a custom usermeta field that i want to search on. i took your code and easily added in the extra field to the usermeta select statement, and it works great.
if you're in the mood to improve your plugin, others may benefit if you gave the ability to add custom usermeta fields names into the plugin config so they wouldn't have to modify the code as i did.
just a suggestion, but i know that maintaining a plugin and adding features is a lot of work.