I always liked the watercolor shading. The photo idea sounds cool as well.The other method that was used was to apply a water colour wash, this could be done from a printed out image, an example from my ever growing "to buld" list may be possible by using very light tones of colour for the various parts and applying model shadow.
The other option for the cover of your build manual would be to take a photo of the finished model and use a sepia or B&W filter, this is a Heinrici I scratch built but gave it a weathered paint finish. There are filters that will make it look like a worm old photo to further add the effect that it is an old original engine
This is another filter that would make it look like an old catalogue image
Thanks!