I think all of those "held" season ticket locations will remain until AFTER the Iowa game. Then those "held" tickets will be made available for single game purchase. Some of the held seats are right by my season ticket location (as they were in years past), and in those past years I was able to buy those seats for some family members who wanted to attend a game or two with us.
I dont know if they were held seats or part of the visitor allotment, but last year, the two seats directly in front of our seats were always either empty or with fans of the other school in them.
These were spots that when I selected my location were not selectable. There were also four open seats in front of the two.
They were prime seats too right on the 50 in the upper deck row 4.