Note: Even public servers that are locked are treated as Private servers, therefore EA/DICE will detect and treat as unranked.
I got this information by thinking of doing the same thing, to get around not being "allowed" to have ranked private servers, and spoken at-length with NFO regarding the issue... no dice, it wont work.
EA/DICE, if detecting a server is password protected, via their own software, turn the server into unranked until the password is removed (I believe the server will then go back to ranked, but you might want to check to be sure about that).
Apparently, they [EA/DICE] are very serious about not allowing any ranked private servers what-so-ever, and there's no way around it.
Just to let people know who are thinking of using the same method as I was, such as dot_Zen mentioned. I'm glad I though enough to ask if this trick would work, and I'm glad NFO was knowledgeable enough to give me a definitive answer.
I don't know why EA/DICE did things this way, but it's really irritating.
To make it more difficult to boost / stat pad I imagine.