There is no reason short of lacking creativity why you shouldn't. In fact we are currently living in Bali where many of the bathrooms are half indoors and half in the garden. I love the sunken garden baths and showers. A sink can be where ever you want it to be. It's all up to you and your imagination. If you like it and it fills a need then why not? We live on a 117 year old sail boat that was a stone carrier for most of her life. Our sinks are hollowed out stones! Sounds mad but the effect is stunning.

I saw a sink/hose reel combo recently. Here are more.

combos

another