OMG… I do desktop publishing and I know EXACTLY what you mean. I never found a solution other than to create an entirely new document and put that one page in landscape and stick it in separately.
Depending on what you’re putting in that page, you can create ‘table’ that has only one cell and you can change the orientation of the printing etc in the cell. But then you’re a bit limited with your formatting. If you have quite a few things to put on your page, you could use text boxes and put the orientation to go bottom to top, so they’re running sideways on the portrait page, too… then you can add text where you want, and even tip pictures on their sides as well in text boxes.
So, essentially, you have to view the page in it’s portrait orientation, but you can put all your content on the page vertically,, with a bit of imagination.
I hope this helps.