I found some old notes.. I looked at this before.
SVN does not actually send the file data during drop. It sends a SVN references and the target must download them self from the SVN Server.
And TortSVN install a shellextension that handle that via the Windows Shell. BUT MC is not built top of the windows shell. (like some other FileManager because of reasons)
So then MC needs to tell TortSVN by it self to fetch the file to the dropped location. Problem is that I did not really find any documentation on how to do that.
Running svn.exe to fetch is might not work, since it will ask for account info, Need to use tell TortSVN to do it since this process already have that info