More flexible layout numbers

At the moment, I am preparing a publication for a publisher, who wants to have a footer with the number of the editon in a special format on every part:

2480 is the number of the editon, 01 is the number of the part and “a” and “b” is for the transposition. (in this case “a” is Trumpet in C and “b” is Trumpet in Bb). The footer has to be on every page of the parts. I can’t think of a clever way to do this automatically, therefore I would like to ask for more flexibitily in the layout number or for another Layout token I can use for cases like this.
Or perhaps I am just blind to see a clever solution for this problem …

If you can get away with using {@StaffLabelsFull@} for the top left corner of the first page of each part, you could use {@LayoutName@} in the footer. It would mean that your right panel in Layout mode would be an incomprehensible list of numbers, though.

Good thinking! But unfortunately not possible for me, because I need the Layout name to show the typical German way of writing the transposition: Trompete 1 in B, Trompete 1 in C and so on.
In the score I don’t want to show the transposition.
(And before you as: I use the full and the short staff names in the score.)