Would it be possible to implement a system to allow the inclusion of pieces with multiple-character IDs in the Diagram parameters that use them?
Which parameters did you have in mind? The promoChoice already allows a comma-separated list of piece IDs, which would remove the ambiguity that could exist if the commas were omitted, and it just test if the (possibly multi-character) piece ID is a substring of promoChoice. Having IDs like +L in Shogi variants is usually not a problem, as promoChoice is not used there, and the pieces are typically not in the initial setup, so you would also not have to indicate them in a shuffle parameter. The latter could be a problem; commas there are already used to separate subsequent shuffles, so if we would want to allow separators there we would have to use something else. Other parameters don't come to mind right away; most commands refer to pieces by sequence number (like royal, or track).
Which parameters did you have in mind? The promoChoice already allows a comma-separated list of piece IDs, which would remove the ambiguity that could exist if the commas were omitted, and it just test if the (possibly multi-character) piece ID is a substring of promoChoice. Having IDs like +L in Shogi variants is usually not a problem, as promoChoice is not used there, and the pieces are typically not in the initial setup, so you would also not have to indicate them in a shuffle parameter. The latter could be a problem; commas there are already used to separate subsequent shuffles, so if we would want to allow separators there we would have to use something else. Other parameters don't come to mind right away; most commands refer to pieces by sequence number (like royal, or track).