No it does not require a string.
To answer your "if Plus! can determine it" question, go to your...
HKCU\Software\Patchou\MsgPlus2\(YourEmail)\Preferences\Plugins and you'll see that all your dlls in Plugins directory are actually "read" by Plus!...My guess is Plus! will first try to load the DLL (and DllMain will be called or something), and if it sucessfully returns true, Plus! attempts to call Initialize, and so forth. If everything's fine, Plus! will recognize it as a Messenger Plus! plug-in.
Smart Patchou, isn't he?