If you don't want to set Mutt as the default mail client so that Opera can call it, I'd imagine that you'd have to write a script/program and set it as the default mail client. Then, in the script/program, detect if the caller is Opera. If it is, call Mutt with the arguments Opera passed to the script. If the caller is something else, pass the arguments to the mail client that you'd normally want as your default mail client.
As in, make a little mailto broker that handles your Opera case and set it as the default client on the system.