When I did Nanowrimo, about 6 or 7 years ago my constraints were slightly different. I needed to work on multiple machines, as I was writing sometimes at work on the dev network, sometimes on the work management network, and at home on my wife's Windows box.
I used a mixture of Abiword, (on the dev network) and Word on the Windows boxen, using RTF as my interchange format. In practice, the extra complexity of the Word interface was not an issue - thinking of something to say, making the daily word count, keeping going in the third week were the problems, not the UI.