We have those windows in our own home as well as a couple of customer's.
If you are doing it WFP then turn the flow rate right down and wash the glass only. You will find you shouldn't have a problem if you are careful. We practiced on our own first before we tried it on our customers, hence the advise.
But again, if in doubt, don't.
Just reduce the price accordingly. Maybe try to get a mobile number to text in the summer asking them to keep the windows closed until you have been. When winter comes they will be closed.
Spruce