[kovidgoyal]

Yeah, unfortunately, as far as I know, Safari prevents webpages from initiating fullscreen mode themselves. The only way to do it is by scrolling, and obviously, scrolling does not work in the viewer, since it is a single screen view.

Your alternatives are basically to get into full screen mode before opening the viewer, or after, using a hack like the one you discovered to enter it after opening the viewer. There may also be some button in safari's settings somewhere to trigger fullscreen mode, but I have not found one (although since I only got to try for a couple of hours when someone lent me an ipad, I may have missed it).