I have a custom piece of paper (4.13" x 8.5") that will print just fine from Tray 1.
I purchased the custom media tray (cb527a) given that its specification sheet says it can print dimensions of 102~105mm to 170mm by 140mm to 282mm.
I've been unsuccessful in printing a 4.13" x 8.5" sheet of paper from this custom tray (Tray 2). The printer is confused when I set up the guides in the positions for this odd size (it's in portrait mode, no different than tray 1). It seems to think the sizing is 6.69" x 11.1".
This custom media tray can take all sorts of ISO sizes as well as common card sizes ranging from 3x5 to 5x8.. It seems especially overzealous for a product labeled as "enterprise" to prevent custom paper sizes when the dimensions are within its specification range...
If you've got this working, it would be a tremendous help if you could lend your expertise to me. I've tried configuring the tray from the device console and via the web administration.. the HP Universal PS driver doesn't seem to offer any help either...