I have a wooden plank (like a flat swing seat) and a hanging chain (or two, like a swing). Instead of screwing an eye into the plank I want the chain to continue downwards through a hole in the plank, so that its height on the chain can be adjusted and other things can hang farther down the chain. Is there hardware designed specifically for this purpose, or a good combination of common hardware to accomplish this?
The naive approach is to just put a bolt through the chain, below the plank, at the appropriate height, possibly with some washers around the chain to sit between the plank and the side of the bolt.
My biggest worry with this approach is how the bolt will interact with the other hanging chain link that passes through the same chain link that the bolt passes through.
Sparr's answer is good except that I will add that you might want to consider putting legs on the bottom with a bolt and cotter pin between them. I found this picture online that is pretty close to what you will want to do.
The cotter pin can be easily removed to shorten/lengthen the chain. The bolt is smooth so there won't be friction there and allows for the chain to move back and forth between the two legs. The only pressure against the pin is horizontal and it should hold.
The pressure on the legs will be equal to the weight on the plank, so be sure to beef them up well if you intend this to hold a good deal of weight, like the weight of a person for instance.