Barnhart, that is on of the places where the 4th edition fell down. Hidden units used to be a
special rule in the 3rd edition. (that is where the 16.6 rule reference comes from.) Now the hidden unit rule is more obscure, but available over at Avalanche Press in old
Daily Content post.
I am pretty sure I know what you are working on. In Vassal, you can still mark units hidden as a solo player, and pretend that you don't see them. There are also other schemes, like semi-random discovery. Someone recently had an interesting way of dealing with the hidden units in Mentoring scenario 3. He recently graduated too, I think, but I can't quite remember.