Is it possible to mount a directory, either an ext# local one, or one in a /media mounted windows drive, so it is seen as a mounted drive itself?
If you only want quick access to a certain directory in Nautilus and the file dialogs, you can also use a bookmark of course...
would result in
The first directory you give is the existing one, the second directory you give is an empty directory under which the contents of the existing one will appear.
To unmount, use:
PS: Nautilus (the file manager) will show mountable/mounted filesystems, not drives, but I assume that's what you were looking for.
If you mount a folder using a transparent fusefs system, gnome will pick it up and show it as a drive. You'll loose some overhead in the calls, but not much for a fast machine.
You could also make a gvfs system that mounts a folder, or a localhost ftp, ssh or other type of mount. That will show as a drive too.