If someone sees you with your bits out are they going to report you for having a pee or are they going to report you for having your bits out in public.?
In that case you would have to convince the police and RM that you were taking a piss and not well....involved in something darker.
If it's a kid that sees you it's going to get messy because anybody whose kid comes home and says they saw the postie with his bits out is going to go off the deep end and create hell with RM who will obviously pass that hell down the chain to you.
Why anyone would take any of those risks is beyond me.
Make sure you have toilet facilities identified on your walk and use them or go back to the office.