It seems that this feature is not working. Whenever I try to make a move in the sample preset, GC responds by saying that the selected set does not exist.
In general, I think this was a bad idea, and if it's not working for you, I would recommend not doing it. The problem with this idea is that it defines a piece set entirely with code, but the preset needs to know how to identify pieces even when not running any code. The alternative, which I would recommend instead, is to use a piece set that includes all the piece images you want to use, then use aliases to specify the notation you want players to use for the pieces during the course of the game. The fairychess include file is set up to use aliases, and the tutorial on it describes how you should make use of them with it.
In general, I think this was a bad idea, and if it's not working for you, I would recommend not doing it. The problem with this idea is that it defines a piece set entirely with code, but the preset needs to know how to identify pieces even when not running any code. The alternative, which I would recommend instead, is to use a piece set that includes all the piece images you want to use, then use aliases to specify the notation you want players to use for the pieces during the course of the game. The fairychess include file is set up to use aliases, and the tutorial on it describes how you should make use of them with it.