The Heliopan ND 3.0 should fit the Lee adapter ring, stray light is almost certainly going to be a major problem however. The best option would be to remove all the 100mm filters slots, thus allowing the filter ring to be attached directly to the holder. Theoretically additional filters (if required) could be attached to the 105mm front thread of ND 3.0. Positioning filters such as grads would be a nightmare however, due to the high density of the filter.
Personally I would either wait for the Lee or purchase a 100mm square alternative. The new Hitech ProStop IR ND 3, is an option. I've used the older Hitech ProStop 3.0, it had a blue cast similar to that produced by the Lee Big Stopper. Hitech claim that the new IR version is the worlds most neutral ND, I haven't seen a review of it yet however.