Hi there,
I've just done this myself and it's simple enough.
First, install Opengoo in the sub directory. Then go to the login page (the one in the directory) and copy the source code (this can be viewed in most browsers: View > Page Source)
Once you've done this create an html file (index.html) and paste the source code from the login into it, then upload. That should work fine!
Alternatively, create the index file and use an iframe to embed the login page
Hope that helps