Is it appropriate to informally address my supervisor in the thesis acknowledgements?
This is going to vary a lot by the country and customs of your university, and your relationship with your advisor. But I'd say, if you relationship with your advisor is good, then in the UK it would not be seen as out of order to refer to your supervisor as informally as you like in the acknowledgements sections of your thesis. In the theses I've read, these things are often a lot like best man speeches - often written in a jovial and cheeky style, while staying just the right side of the appropriateness line, but also often cloyingly sentimental at the same time.