There is no way to tell if a file has been locked by someone else, so I need a new overlay to show that.
Unfortunately, that's not possible. Locking information is held in the repository, so in order to refresh the page in explorer, we would have to contact the repository. That would make explorer impossibly slow. Servers often take several seconds to respond, sometimes minutes - do you really want explorer to hang while that takes place, every time you open a versioned folder?
A fundamental design feature of TortoiseSVN is that the repository is never contacted except when explicitly requested by one of the context menu items. Even with that restriction, it is still hard work maintaining a fast response.
If you use locking to prevent simultaneous editing, then you should consider using the svn:needs-lock property. Read the locking section in the manual to learn how to use this property. That way, files in your working copy will be read-only and have a grey overlay until you acquire a lock.
If you want to see which users have files locked in their working copies, use the Check for Modifications dialog and click on the Check repository button.