When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server, the client must send only "*" (%x2A) as the request-target.
OPTIONS * HTTP/1.1
curl -X OPTIONS http://example.org
OPTIONS http://example.org/ HTTP/1.1
(speaking as the main curl developer)
I honestly don't think it is possible (that is, using
-X OPTIONS etc will send a slash and not an asterisk), although I would of course be very open to making it possible in a future version in case someone wants to join in and help us fix this oversight.