I think he'd like these admins to be able to do their job whether or not they're in the domain admins group or not. Assuming this, I know the OU's will work for what he needs. Like I said previously, I'm not an expert on encryption so I'm not sure how easy or difficult it would be for a domain admin to break in.
Either way, I know OU's would work in this situation. You seem to know that PGP encryption will work too. There are almost always different strategies to accomplish the same task.
So why argue? We all gave some good advice here.
Take care!
:D
