There's a well known (former? lurking?) member here that would "scan" his films this way.
My question would be 35mm slides of which I shoot basically none these days anyway, but have loads of. They can be very dense, and I wonder how much shadow detail and noise you could get with a top quality dSLR vs my Nikon scanner. Maybe I'll have to try some shots comparing my Coolscan III to my D300s? My old 55/3.5 Micro-Nikkor should kick butt for this type of work.
There is only one way to find out. Try a simple set up. 1 See how close you can get to show a full frame image.
2 Roll a card tube of that dimension to fit over the lens. 3 Fit a 35mm neg to front of tube using card mask, sticky tape, etc. 4 If you have a white diffuser over the neg. If stuck for a diffuser use a piece of thin white plastic from some product in the house. 5 Check focus, set to best f number, auto focus and click.
I like the live view camera approach to make it easy to see focus is ok, neg is centred, etc.
Please try and report your findings here with comparative pictures.