The research for this projects feasibility has been hindered by the high cost of 3.75v nominal lithium cells. even the consumer "d" size lithium cells are too expensive at the quantity needed, I cannot get the price lower than $250++/7.4v/100ah 2012 module. the problem is that buying at end consumer quantities it's hard to get the price down to what i am willing to pay. the inability of business to sell (beyond pre orders that are endlessly delayed) supplemental and replacement packs speaks to the difficulty of this. the next problem is that the NMC battery chemistry, and its higher cost, are becoming more dis-favored in the market place so innovation is not as high as LiFe and LiTi, which is driving the prices down on these chemistries. there was some of papers on saving higher voltage Lithium cells at SAE congress this year, one was using fluoridated electrolyte, one using dynamic charging strategies, but these 'future' battery techs do little to save 2011-2013 leafs that are getting to the end of battery life in the next few years.
for an unrelated project i'm look at 60mm diameter 280mm long cylindrical LiFePo4 cells that are priced lower than lead acid, and if they are not fake garbage (which they probably are) then maybe that is a way to go, either as a 110-series pack replacement to reclaim the traction pack space, or as a pack supplement with >100% of the new pack capacity. also the pusher trailer idea has merit, either with the LiFe batteries or a propane-fueled generator.