When I was doing the repotting of my DRs in April I did some root pruning as well. I had cut off a couple of undesirable roots that were either in the way or eventually would not add to the over all appearance of my oldest DR. Rather than just throw away the decent sized roots I wanted to experiment with grafting. Unfortunately I didn't take any before and during pictures but here is the result of the experiment.
This root was at the very bottom of the DR growing between two other nice fat roots in a horizontal path. I was worried it'd grow in such a way to trap dirt and such that might could eventually lead to rot. Besides, it was underground so it wouldn't never really see the light of day. I did the flat (slightly angled) graft and then planted it vertically. I'll be curious how this root develops over the years.
I should add that the grafts were perfectly flat. While it was bonding the scion pushed down to form that weird gap. I'm guessing because of young tissue growing faster than old tissue of the root. But it's filling in fast as you can see by the last two closeup shots.
So if for some reason you have a need to prune off some big roots due to rot or what ever I suggest saving them cleaning of the rot and do a graft. What do you have to loose?