It certainly seems possible to enginer some device that could be inserted into the riser and expand/inflate to form a seal against the inside of the riser pipe. If they can temporarily overcome the outflow pressure by pumping mud, they should be able to force a device like this down the well bore
I actually had a different idea that wouldn't risk damaging the pipe more, as something that expands and seals it might.
So, basically take the first idea they had with that huge metal box (the one with methane crystal formation), but instead of it being a metal box make it out of concrete. So, there is a massive concrete box that then has a cone/inverted funnel shape inside that goes up to where three hoses (three for redundancy) is attached to suck the oil out as it leaves the pipe.
Also, I'm not familiar with how methane crystals form, but as I understand it a major cause to their formation is the cold temp. If that is the case, why wouldn't a simple heater coil work (like an electric stove coil and electric heaters) imbedded into the steel prevent ice from being formed? Basically like a heating blanket that keeps the steel above the temp the crystals form at? Maybe I'm missing something here, anybody who has more knowledge on it care to comment?