.. I found the answer..
Alas, the User Manual (HP doc: c02677203) has crushed my dreams on Page 73/74.
Short version, it's not possible to print my 4.25" x 8.5" document via the custom media tray in tray 2. The minimum custom size for Tray 2 is 5.83" x 8.27". So very disappointed for a product labeled as "Enterprise".