Mostly spraying just happens. If you have enough bees around, particularly at a distance you won't be able to get round to them all anyhow. Ideally spraying will take place in calm, no wind, situations, early or late in the day, when there are relatively few foragers abroad. Also most spraying of rape takes place when the crop isn't in flower. The exception to this is when spraying for leaf drop.
If you have an open mesh floor, then closing the entrance should hopefully just stop ingress and egress from the hive, not stop ventilation.
Provision of sufficient super space for the foragers should ensure that they don't overheat if you do close them up.
I'm sure that others more qualified than myself will advise you further. It's always good to be talking to the farmers as they are more likely to tell you these things in passing conversation.