Well done, Harvey.
I also noticed, that under Win7 you don't get the Printer selection button in Mathcads page setup dialog window. So probably setting the pdfprinter as default as you wrote is the only way here.
I just tried your proposal, but I used a page of double height instead of half width. It worked OK but I just noticed that Mathcad will make the page breaks as it would be a normal sized page and not double height. Not sure if io made something wrong.