I think that ControlTitle_.Text is supposed to work; There are a lot of entries for it in the resx files in the DNN source.
The function that was used prior to DNN 5 (I'm working with DNN 5 to test this stuff) was Deprecated, and in the new function, it has the line
If String.IsNullOrEmpty(controlTitle) AndAlso Not String.IsNullOrEmpty(controlKey) Then
Where the 'controlKey' variable is going to be blank, making the localization not take effect.
LocalizeControlTitle in Localization.vb @ line 1422 is where I pulled that from.
To note - I spent 5 minutes on this, so I could be wrong, but... ControlTitle_.Text didn't work for me either, and I checked the DNN 5.1.2 source I have for it, found it lots of places... And then found that function. I could be entirely wrong is what I'm getting at, it just looks kinda like it's specifically excluding the blank controlKey, where as the previous function does not appear to have done that.
Excellent overall question though, I did not know about it, and am now planning to use it in many places.