I agree I was working a bit on doing that today.

I made an extension which adds support for this. It modifies War3Source_Engine_MenuChangerace, so it will have to be reimplemented every time there's a new release, but it works well. I admit the method I used is a bit silly, but I was trying to get it done quick without a lot of work. I basically highjacked the part of the code which checks if admin flags are assigned to a race, and added a special case for the flag "p" (private, get it?). If the race has that flag on it, it checks a keyvalue structured file for the user's steamid under that race's shortname. If the user's ID is not found then the player does not have access to the race. This method allows one to make a race private on-the-fly, without having to restart the server or change map, but doesn't guarantee that players who are already playing as the race will be switched to a different race if they are not included in the permissions file.

A better way to do this would be to create a race cvar like <shortname>_isprivate <1/0>, and have the code check the keyvalue file if it is set to 1, but that would take more work for me to re-implement every time there's an update (more code separation). Since this is such an easy thing to add, I don't see why it isn't implemented officially. I also suggest checking for permissions on player spawn to make this work well with on-the-fly changes.

If anybody wants the sp file, I can upload it.

This is true, but I wanted the user to be aware that a race was private when he accesses the changerace menu. This meant I had to modify the changerace menu code itself. I couldn't simply hook into the race selection event, even though this would allow partial implementation of private race support. As in, if a player changes to a race, check if the race is private, switch him to different race if he's not allowed to use it. That, I figured would be annoying because the player is never made aware that the race is private until he tries to select it, then he has to go and open the menu again and find another race.

mysql would be even more complicated

Well, this one would be for private use XD So we don't really bother with supporting anything other than MySQL