Unfortunately this is not possible yet. The custom field filters only work with custom fields, which is the wp_postmeta table – and it connects with the posts database table. Even if you changed the appearance to display the user meta table contents, it still would not work – because on within the search algorithm there is no handler defined to make the connection between the users table and the user meta table.
I cannot suggest a quick fix for this, as it's much more complicated than that, but it's a planned feature, which I'm implementing as soon as possible.Best,
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