There may be a better way, but all I can think of is to use the user role permissions and for each role necessary, set the field permissions so that all fields except for the one are view only.
Then in the table access permissions, do a custom rule for modifying the record, so that the record itself can only be modified under your condition.
This way, the record as a whole is locked until your condition, and even when unlocked, only the one field is set to be editable. You would need to do this for all roles you want this behavior for.
