Replying 7 years late, but others may find this useful. Anything inserted into the hot shoe that isn't a recognised flash unit (e.g. a plastic hot shoe cover or an external viewfinder) causes a red "Err" to appear in the built-in viewfinder when 'P' mode is used for aperture control. So, no... an external viewfinder cannot be used if you also want to use 'P' mode. I haven't tested it, but if you're happy to use manual aperture selection, then the "Err" message doesn't appear and the shutter fires when, for example, a plastic cover is in the shoe.