Well, let's see. Assuming Nebraska kids are like any others in the nation in their desire to go pro - or at least have a shot at it - they'll want to go to a program that teaches pass blocking skills. Now, they may not come into the program with the level of pass-blocking skills that kids from states that have more passing offenses will have, but that's part of the evaluation process - can a kid be taught the skills? Does he have good feet? If so, then they'll fit the system. They may take an extra year of coaching, but physically they have the skills.
Sure, if the coaching staff can recruit players that already have more of the skills, they'll do that. But it seems that most linemen tend to stay in the geographic region more than skill players. That means that Callahan and staff may not be able to recruit the numbers that are needed outside the mid-west in general, and Nebraska in particular.
Finally, high schools tend to eventually run the offenses of the major college in their region. Over time, we'll probably see more high schools going to more open offenses, which means that more linemen will have the same basic pass-blocking skills that are found in linemen in other states.