The only thing the diagram uses the piece IDs for (apart from printing, for which they could be anything), is to check which promotions are allowed, by testing whether the ID occurs in promoChoice. When I wrote the code for that I imagined piece IDs would always be single characters, so I did not take the trouble to require any separator characters between piece names in promoChoice. E.g. for Chess I it would just be NBRQ . It just looks whether the piece ID is a sub-string of promoChoice. This is why multi-character Ids can give problems, expecially when mixed with single-character Ids. If all IDs were two characters, and they were separated by spaces (or whatever punctuation) in promoChoice, there would not be any problem. Only if the ID of one piece would be a sub-string of that of another it could be confused, and even then it is only a problem if you want to allow promotion to the long name, but not to the short name.
In Shogi variants promotion is usually controlled by promoOffset, which defines a fixed promoted form for each promotable piece. In that case promoChoice is ignored, and there never can be a problem, no matter what the piece IDs are. This is why the Shogi custom of indicating promoted pieces with a + prefix to the unpromoted ID is never harmful.
Using a * or ! as prefix in a piece ID is the most problematic of anything you could do, because these symbols have a special meaning in the promoChoice string: a *L there would mean you can promote to L, but only if you have an L in hand (usually because it was captured before). Other prefixes (e.g. #L) would probably be harmless, as long as you don't want to use them to distinguish an L from a #L, and want to allow choosing a #L but not an L.
The only thing the diagram uses the piece IDs for (apart from printing, for which they could be anything), is to check which promotions are allowed, by testing whether the ID occurs in promoChoice. When I wrote the code for that I imagined piece IDs would always be single characters, so I did not take the trouble to require any separator characters between piece names in promoChoice. E.g. for Chess I it would just be NBRQ . It just looks whether the piece ID is a sub-string of promoChoice. This is why multi-character Ids can give problems, expecially when mixed with single-character Ids. If all IDs were two characters, and they were separated by spaces (or whatever punctuation) in promoChoice, there would not be any problem. Only if the ID of one piece would be a sub-string of that of another it could be confused, and even then it is only a problem if you want to allow promotion to the long name, but not to the short name.
In Shogi variants promotion is usually controlled by promoOffset, which defines a fixed promoted form for each promotable piece. In that case promoChoice is ignored, and there never can be a problem, no matter what the piece IDs are. This is why the Shogi custom of indicating promoted pieces with a + prefix to the unpromoted ID is never harmful.
Using a * or ! as prefix in a piece ID is the most problematic of anything you could do, because these symbols have a special meaning in the promoChoice string: a *L there would mean you can promote to L, but only if you have an L in hand (usually because it was captured before). Other prefixes (e.g. #L) would probably be harmless, as long as you don't want to use them to distinguish an L from a #L, and want to allow choosing a #L but not an L.