This does seem to work reliably: (replace myuser with the user you want to unhide.)
sudo psql -U _postgres collab << EOF
select * from entity where is_hidden=true;
update entity set is_hidden=false where short_name='myuser';
update entity set is_hidden=true where short_name='unauthenticated';
update entity set is_hidden=true where short_name='authenticated';
EOF
wikiadmin resetQuicklooks
wikiadmin fixPermissions
wikiadmin rebuildSearchIndex
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The select statement is there just in case you need to revert something, you have what the previous state was.