If you haven't looked at them before, the Rhodia dotPad or Top Wirebound Notepad might be a fit for you and usable in either orientation. Rhodia's "pad" products seem to all have perforated pages (they tear nicely) where the "book" products are non-perforated.
Edit: the wirebound has an A4 size page option, the dotPad has an A4 and a longer-than-A4 option. ~$10-16 depending on color and where you order, Amazon is not the cheapest option out there.
The longer-than-A4 option is intriguing. I’m wondering if a print shop would be able to cut off the top binding and change it to a glue edge along one of the long sides?
