Hi Mick,
I might be missing something but can't you use the secondary vane adjustments to centre the mirrors in respect to each other?
I have a full thickness newtonian sitting in a four point cell - very easy to build. Laterally the mirror rtests against 3 plastic poles. This seems to work fine. Full thickness mirrors are fairly forgiving.
On a cassegrain you could provide the lateral support via the baffle tube but if you do so, read this first:
http://www.atmsite.org/contrib/Sapp/Baffle/
Good Luck!
Rod.