This is indeed something that cannot be done through the Applet, but which can be specified in the HTML for an Interactive Diagram. With the promoChoice parameter you can specify several 'promotion sets', separated by slashes. For each piece that promotes you can then specify a morph parameter to indicate where it promotes, and a digit 1-9 would indicate which of the extra promotion sets should be offered as choice there. (For the first set the square in the morph value should be set to *.)
This is indeed something that cannot be done through the Applet, but which can be specified in the HTML for an Interactive Diagram. With the promoChoice parameter you can specify several 'promotion sets', separated by slashes. For each piece that promotes you can then specify a morph parameter to indicate where it promotes, and a digit 1-9 would indicate which of the extra promotion sets should be offered as choice there. (For the first set the square in the morph value should be set to *.)