Okay it's getting annoying now so for the first time I thought about a solution myself. Since there's no textbook solution, it seems that you need some fantasy to fix this mess FAST!
So what I don't know is: What does it look like down there? Is it a pipe sticking out of the ground? How much of it sticks out?
What seemed most reasonable to me was heavy equipment down there. So I imagined dropping a tank (heavy, solid and reliable) first. No, not just dropping it hoping it would land on the leaking pipe, of course! You can just place it somewhere close to the site. Then I thought maybe an excavator would be better since it's also heavy and already has hydraulic pumps and stuff on board to power attachments needed at the site. And finally I came up with this idea: A one week plan to design and build a whole new machine specifically for this job! Not doable? I would say that although it seems hard it is possible with the right people involved. The design doesn't have to be pretty or sophisticated. Weight doesn't matter nor its looks. So designing would only take one day. Yes, including blueprints ready to be sent out to the shops. The parts have to be done in 5 days and the assembly in the final day. Tough job but as I said, with the right people you can do this! I'm a machinist and I see funny things sometimes. So the design has to be done by people who also know how to machine parts. Reasonable tolerances and machinable design. It can save DAYS not just a few hours in machining time if the design is right.
Keep it as simple as possible. All you need is a big frame with 4 wheels on it. Maybe an independent suspension or maybe no wheels at all are necessary. I don't know what it looks like down there. Perhaps you can just place that frame next to the leaking pipe. A battery powered electric motor powering the hydraulic system for basically every function of the vehicle.
Don't you think that with a massive machinery at the site down there it would be easy to work on this pipe? You'll have a steady piece of equipment to cut the pipe and mount whatever you want to stop the oil flow!