Interesting stuff remou
I've done this before, my technique is probably kind of clunky in comparison.
I just lock all the fields On Open except the Lock Flag field and the Combo box. Then use an after update on the Lock Flag with an IF statement to unlock/lock fields. You can also work this into the On Current, After Update, ect..., depending upon your situation
The problem with just a blanket lock is I've always had trouble unlocking as the form is locked, ect..