I've just encountered reports of your work via Wired.com and Slashdot.org and wanted to send some words of encouragement and a couple of thoughts.
First of all - well done! I wish I lived in NZ so I could come and help with the project.
Secondly, I've been researching the construction of astronomical markers for a number of years, and have come to a realisation that the date of the equinoxes can be found with *no* lengthy observations necessary and only:
1) One stone obelisk
2) One wooden pole
3) One flat reflective surface
Step 1: erect the stone anywhere
Step 2: place the wooden pole in the ground due South of the stone (due North, in the southern hemisphere) - using the stone as a backsight and the pole as the foresight this is simply achieved any clear night by lining up pole -> stone -> Polaris in the northern hemisphere
Step 3: attach the mirror to the pole so that it faces the stone
Step 4: twist the pole and attached mirror through 45 degrees clockwise (anticlockwise in the Southern hemisphere)
Step 5: when, at dawn, the stone is centrally illuminated by the reflected rays of the rising sun, then it's an equinox
Interestingly, if you were to be on the Equator, then using this method you could at any time position the two solstice marker stones as well as the equinox stone through the use of a 30 unit long rope marked off into lengths of 5, 12 and 13 units (Pythagoras right angled triangle values).
To do so, you choose to position the wooden pole the full length of the rope (30 units) away from the single standing stone. Then you measure 13 units to the left and right of the equinox stone and place the solstice stones there.
This happy situation arises because arctan (13/30) is roughly 23.5 degrees (23d25m43s), being - as you'll realise - the approx axial tilt of the Earth and therefore the maximum extent of swing N and S of east of the sun on the horizon at 0deg latitude.
The ancient Egyptian ceremony of the "Raising of the Djed Pillar" was timed for the spring equinox, but the significance of the ceremony in terms of the knowledge it demonstrated is not understood. I suspect that it was the ceremonial raising of the wooden pole and reflective surface component of this basic marker system.
Knowing the date of the "balance of the year" was a critical skill for any culture dependent on recognising the start of the growing season and the ability to *extremely* simply erect a means of determining precisely when the sun rises due east using a stone, a stick and a reflective surface without any complex surveying might even have been marvelled at.
Naturally, the distance from equinox stone to solstice stone varies according to latitude but one year's observation serves to discover the lengths necessary to use.
Perhaps the mystery of Stonehenge's alignments would have been more easily solved if the pretty, and valuable, reflective surfaces that I believe must have been hung on various stones at various times of the year hadn't been stolen sometime in the last 4500 years :-) The StoneHenge Bush Barrow lozenge would be one example.
Everyone seems to think that alignments must be laid out by standing somewhere and waiting for the sun to rise over a landscape marker, followed by putting a stone in place as a foresight leading to arguments about whether the UK StoneHenge solstice marker is over the Heelstone, between the Heelstone and some other stone (now lost) or something else. Whereas, in fact, if you've got a mirror in the right place then you can be as precise as you like (and you've only got to move a wooden pole), and can operate your model calendar at any scale you find convenient, from a few inches to hundreds of yards.
Hope this email has, at least, been an interesting distraction for you. Best of luck with the project itself - along with the Clock of the Long Now and the UK's Solar Pyramid projects this is a wonderful way to engage people with the rhythm of the years once again.
Can I recommend that you carve all the intended alignments and a plan of the site's design on to a diorite stela and bury it in a magnetic box underneath the central obelisk, just to avoid another set of arguments about what it was all for in 4,500 years time? :-) A modern stone circle at the UK's SpaceGuard observatory in Powys, Wales, erected a few years ago, marks (amongst other things) the rising of the direction of the centre of the galaxy and that of the rising of Cygnus X-1 - and that's going to confuse the heck out of any archaeoastronomers of the year 6,500!
All the very best,