Yes, that sounds plausible,
Do you have a simple dual voltage secondary transformer? If so why not give it a go with just two voltage outputs and switch between them?
I'm sure you know what I'm suggesting, but for others, mains-fuse-transformer voltage in 240V-transformer voltage out 6/12V-rectifier to DC,
The outputs will be slightly higher due to being rectified but close enough. I really don't think the actual speed of the wiper motor is that critical from my exp, just so long as you have a fast and a slow speed for convenience.
I must rig this up sometime and test it in real life as I promised to sort something out for Stew. But I think it will work fine and would be rock solid reliable (unlike fancy electronics)
Just a suggestion.