if you was in the bay you could do it and it would drain through the holes in the floor of your rusty english bus.
Can you not get down on your knees and do it somehow in the windscreen washer water container, or disconnect the container and do it then? I've never heard of anyone doing it before but I do remember briefly galancing down at it yesterday and thinking 'that could come in handy', for the purposes you mention now.
Thought it was why buses had sinks in? Or is one not in a bus? Hmm window? Hard shoulder? Pick a bush, any bush!