I was able to reduce the slack on my FEC freecoaster. Thought I'd share what I did. I can't comment as to the durability and possible problems with use. I've only got a few hours on it like this but its reversible and I can't see how any permanent damage could occur.
I found a scrap o-ring that had a cord diameter of 0.100". I cut it about 3/8" long and put in in the groove as shown in the picture. This reduced the driver rotation until it hits the piece that slips on the axle. At first I cut the o-ring about 9/16". The freecoaster still worked 100% but the slack was almost zero. Way too big a different for me to cope with after having the stock set-up. The 3/8" length gives me about an 1/8 turn on the crank before engagement.