If it's raining hard, it's rare I clean because Wor Lass complains that I'm a task master and I doubt I could spent a whole day soaked either.
But I will clean in the rain if I'm behind, and we always clean in the often drizzly weather we get in South Wales.
You don't have to be crafty. Just turn up, all professional like; don't even mention the weather.
Clean the windows, knock, say, 'Good morning' and collect what you've earnt!
If anyone gets funny (which has happened ONLY ONCE) to me in 2 and a half years, just politely explain that you've got that much work on, if they want their windows cleaned regularly, they get cleaned every month; no matter what the weather is like.
I asked the same question about cleaning in the rain about two years ago. It might've been Lionheart (I'll have a look now) explain to me that all I required was some 'balls', and once I acquired them it wouldn't be a problem. He was dead right.