We are in a similar situation with high mix low volume production. We are also using lean principles and the one piece flow mantra is always the target. In our plant though, we have not been able to achieve it. The guys who want one piece flow mostly come from volume thinking. The idea is elimination of the waste of waiting and inventory. But changeover is a waste and so is waiting. As a result, we ended up moving more towards controlling the WIP level at any queue locations. We have a few SKUs where we do come close to single piece flow from SMT to the end of wave, but we still needed a buffer after that so the end cell doesn't go down when the board building are runs other products. I'll close with this warning, don't let the lean guys ignore any of your costs and if your intuitions says they are asking for something wrong, challenge it but be open to a new way of looking at things.