I would encourage you to position the tang for your runners as far aft as you can. I have never heard of a "symmetry" applicable to distance forward & aft of the mast in this regard.
The farther back you can position the tangs the more the load on the runners will be in countering the tendency of the mast to "pump" or ultimately bend. As you seem to know - the objective is to stop a buckling of the mast. The force vectors yield more and more to tension (up & down) as the runner attachment points move closer to the mast. You need to counter a force on the mast that will be largely longitudinal - fore & aft.
As far as annoyance, or inconvenience of using runners while tacking or gybing... yes, that is the nature of the beast. Letting the tangs reside forward certainly does open up the arc in which the boom can swing, but it also reduces the protection against a pumping mast - a poor trade.
ps as a Maine based sailor I defer to your better knowledge of Fundy. I do have to add that my one sail on it in the summer of '07 was a delight.
Last edited by wwilson; 11-24-2009 at 06:02 PM.