I haven't put many cameras directly on scrapes, but have occasionally. I just put it right on the tree that the scrape was under looking straight out at the scrape (about 12-15' from the trunk to the scrape). This was a natural scrape that gets opened each year and has been there several years. It is also in a break in trees in an old fence line where deer somewhat funnel thru, so I got pics of deer traveling as well as stopping to use the scrape. Maybe should mention this was with an IR camera, so no bright flash. Not sure if a flash would bother. Also, most of the action happened during dark.
I've also tried the mock scrapes with a scrape starter and scrape drippers and got some response (deer tracks or scrape marks in the scrape), but never had a camera on the tree to see what was there. I wouldn't say I ever got a lot of use out of the mock scrapes as the natural ones always look more heavily used to me, and it got costly to keep the drippers full.