You havn't said what OS you are using, but I take it from the mention of Notepad++ its Windows. IIUC you can set where to install Geany using the `setup.exe` installer, so you can re-install Geany in your own directory, then install geany-plugins to the same place.
But AFAIK there is no guarantee that there are not libraries that need to go to system directories, so if the above doesn't work there is a possible option B. IF (big IF, I don't know Windows requirements at all) the plugin needs no libraries other than what Geany uses then you might be able to just copy the dll from the `setup.exe` using an archive manager. Even on Linux I can read the contents of the `setup.exe` and navigate to the `lib/geany/projectorganiser.dll`. As best I can tell its the only thing projectorganiser needs, but the installer has a lot of system libraries that it might need.
There is a setting in `Edit->Preferences->General->Startup->Extra Plugin Path` that you can set in the system installed Geany to point to a non-system directory you can write the `.dll` file to. Hopefully it will work, but no guarantees on Windows.