I think what these pic's show are acts of complete desperation by a deluded dictator who vastly underestimated the resolve of the "Coalition of the Willing". I don't think Saddam actually expected a full on invasion, yet still took these desperate measures, hoping at least being able to assemble some kind of rag tag air force once it was all over. In a way, he had no choice but to bury them hoping to get at least something back, versus having them completely destroyed either in the air or on the ground during GW2. Saddam got none of his air force back that went to Iran during GW1, so I guess burying them is the next best thing huh!
Having worked in places like quarries and glass plants over the years, dry dust and dry sand can really get into well sealed enclosures. Dry sand flows like water. Still, nothing is impossible. If you know the basic empty weight of the aeroplane beforehand, stripping the aeroplane down and giving it a good vacuum until you reach that BEW would get you pretty close I'd think. The main concern I think would be structural overload - putting a lot of dirt on top of the aeroplane - what overload and damage did that do? The MiG-25 is a pretty tough aeroplane made mainly of nickel steel, so if any aeroplane can be buried and brought back to life, this would be it I think. Electronics are pretty basic being largely old valve technology. Who knows - bury five and hope to get one or two going? Desperate and crazy stuff indeed!
Not a bad explanation of what went where and why - if this Wiki source is to be believed.
http://en.wikipedia.org/wiki/Iraqi_Air_Force