In the areas of the US with a high microbrewery density some small companies have popped up who have installed bottling lines in semi tractor trailers and they simply drive the bottling line to each brewery on a schedule and contract bottle at each brewery.
-Looking at the recent wave of breweries shipping beer in bulk to England and then bottling it certainly seems like such an opportunity exists.
Or even for a few breweries to start a legal partnership on the side to own a bottling line and share it. (Not sure if we're quite there yet, but I'm just sharing lessons-learned from across the pond.)
Adam