It would be nice if WinSCP could, by default, use the default Editor for a particular file type without further configuration.
To determine the default editor for a file type, look in HKEY_CLASSES_ROOT/.<extension>
That value (default) will tell you what file <type> it is.
Then look in HKEY_CLASSES_ROOT/<type>/shell/Edit/command
The value of that key will tell you the value of the "Edit" command from Windows explorer's perspective.
A default can be set for all file types, too, so if the file type searched for was non-existent, use the value of HKEY_CLASSES_ROOT/*/shell/Edit/command
From here there shouldn't really be any need to know if an editor is MDI/tabbed, and it should allow WinSCP to work well without separate configuration (and allow it to dynamically update with Windows as necessary).
I hope this helps. As always, WinSCP is an unbelievably fantastic app. Your efforts are highly appreciated by a very large number of people.
When you put everything into something, it means everything to you.