ePub is fine for single column novels and such, but anything above that is just asking for trouble. It's why you don't see any magazines in ePub format.
Even if you OCR, if the text is "burned" on to the image (ie. it's not selectable, in a layer above it), it's very difficult to extract and edit the background to remove all that text. You'll need some serious photoshopping skills. Plus, you will have to track down the fonts or use fonts that look similar. It's a very time consuming, colossal work. But the end result, if done right, is a always pleasure to read.
I imagine an RPG manual written for a fixed layout would have tons of references to page numbers ("see page X"). How are you planning on dealing with those?