Even if Pakistan was willing to risk being caught red handed and allowed a few Chinese personnel onto the air base discretely, those engineers would be able to do precious little just looking at the equipment. There is not even a need for Chinese personnel to physically be at the base if they just wanted a look-see under the hood. The Pakistanis could easily take extremely detailed photo and videos themselves and pass that on.
The issue is that in order to get much useful information and technology, the equipment would need to not just be disassemble, but more importantly, tested. All the critical components are also 'black boxed', and if they are opened, the US would know about it next time they do a check of the planes (assuming the black boxes themselves do no contain sensors and transmitters designed to alert the US instantly if they have been tampered with).
The US are not fools, and have spent a great deal of time, effort and money to make sure their technology is as secure as possible.