I believe they have already attempted to utilize 2 different sizes of "inverted funnel" device, and still have some plans for a third. It is a pretty simple idea, simply build something big enough to cover the leak and heavy enough to stay in place, then drop it over the leak. From the pictures I've seen the top of the funnel device with it's various ports and valves could be pretty generic, with only the lower portion needing to be customized with cut-outs/ports to fit the site. As this approach was presumably planned as the first line of defense in a crisis, I don't think it is unreasonable to have a few of these sitting around just in case.
I recall an awful lot of noise being made about the fact that NO ONE had ever tried the funnel devices at such depth. It turns out they don't work very well when under great pressure. Had someone looked at the containment plan and said, "Gee, no one has ever actually tried to do this, the plan can't be approved without a successful test run.", I don't think anyone would have been surprised (I'm sure some people involved were not surprised) by the problems caused by methane hydrates, and a better option would have been necessary to get the containment plan approved.
Captain, you are correct that in an emergency you never know exactly what you will need to deal with. However that is no excuse for failing to prepare. From what I can tell, some CRITICAL elements of the containment plan are nothing more than a plausible work of fiction. That being allowed to happen is a huge failure on the part of the oil industry and the regulating agencies. I doubt it will be successful, but I think those impacted by the spill should also sue the government regulators for their failures in this situation.