It turns out this is “only” working with variants and not with custom instruments !!!
judging from the release note it should be working for Custom instruments :
But the result is as follow (piccolos are variants and saxophones are custom instruments based on the same “Dorico default alto sax” parent instrument) :
I would have liked to build my entire Custom Instrument Library using the elegant Variant concept but Variants still aren’t capable of saving dedicated Endpoints as you mentioned here :
Since I don’t have a choice but to create Custom Instruments I was really Jumping for joy at your announcement !!