Is it possible for mu metal to block affects from a great deal of metal to shield an experiment using extremely weak magnetic fields down to a pico gauss?
The most effective magnetic field shielding is achieved by high permeability alloys. It may be that to reach such residual magnetic field inside specific space you have to enclose this space with a box having multiple layers of perm alloys or mu metals.
You may get more information about magnetic field shielding from the link:http://www.magnetic-shield.com/faqs-all-about-shielding.html
Without knowing exactly what you are trying to do it's difficult to advise but you may have to consider the magnetic field noise level in your experiment space. The shields will contribute to this.