But the flow out of that well is too much to try that. Besides, while I don't think that is what they used for the top kill attempt, it is one of the "muds" that have been used for similar purposes. According to what I have read, they used a barium based "mud" for this well because of the heavier specific gravity. It did not work because of the leaks from the casing below the BOP and the much greater flow rate than anticipated.
And that gets us to why BP is no longer trying to shut off the well from the top - many experts do not think that the integrity of the BOP and the well casing will survive the pressure if the flow is simply stopped at the top. The consensus now seems to be that could make the entire well head blow out which would make the situation even worse than it is. That is why they are trying to use caps that will collect most of the oil, but not put back pressure on the well.
Go to The Oil Drum,
http://www.theoildrum.com/ Go back a few weeks and read some of the technical discussions. There are all kinds of discussions of the technical reasons why the top kill did not work, what they used, why they have had to give up on stopping from the top and so forth - many oil guys over there that know their stuff have been talking about this for nearly two months. It is a great education.