They have absolutely nothing to lose by gradually raising the back pressure to see what happens as they are ultimately going to need a higher pressure on the wellhead to contain the mud from the relief well. The hydrostatic pressure on the casing will be less when filled with escaping oil than when it is filled with mud. So check it out and cross another worry off the government overseers' worry list. This whole process has been governed by the most timid regulator from the start. For example, instead of screwing around with Leak 3 at the end of the riser, they could have cut off the riser at Leak 2. The big kink was still there to limit the flow! But NO, the guiding principal of the day was "First do no harm", but as was said above "you've got to squeeze some lemons to make lemonade".

That's the problem with putting a Nobel Prize winning physicist in charge and expecting to get Richard Feynmann. People tend to forget that Feynmann's thoughts were only included in the Challenger Commission Report as Appendix F http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/rogers-commission... after he threatened to resign from the commission and subsequent to his o-ring in ice demonstration. This whole administraion is based on using appeals to authority, men & women in fancy uniforms (and Harvard degrees), exactly the kinds of guys Richard Feynmann's father, the uniform salesman, warned him about.