Well, in answer to my own question, I tried my DIY SFX rear filter and it worked rather well (at least the negs look good). On the down side - and a pretty big down it is 'n all - there's an awful amount of faffing around to do before you get the shot. Firstly, you've got to compose the image, make a note of the focussed distance, meter it and apply compensation etc. Next - and this is the real ballache - you've got to remove the lens, remove the rear filter and replace it with the SFX filter, remount the lens and set the distance
plus focus shift. Other than that, it's dead easy
. Maybe I'll post some images when I print them.