There is a NZMCA park on the northern edge of Martinborough that you could access, the site fees are quite reasonable.
Some stallholders camp in their caravan on the road around the square somewhere near their stall site on Friday night so they can make an early start to setting up. (Please don't camp in the grassed area of the square)
Booking of more formal accommodation needs to be arranged well ahead due to the popularity of the Fair. Many late-comers find they can only find accommodation as far away as the Hutt Valley or Masterton.