I have had extremely limited experience in this area, but I can tell you what little I know..
I have a bastardized rg550 that I bought new back in 88. Ive screwed with that thing so much its not even funny. I wanted to try the direct mount thing as well. It has a Duncan distortion
in the bridge, and a jazz in the neck. (Both sound awesome) I took some small sheet metal screws and screwed them through the pickup mounting tab holes, and into the body. Since the pickups would sit too low if mounted hard againt the body, I kept small springs on them, to keep it pushed up. I suppose you could glue a small piece of wood into the little side rout (For the mounting tabs) to raise the pickup to the desired height, but I just screwed them in as is. So they arent "Hard" against the body, but they are mounted to the body. It is definitely more solid then hanging from the pickguard. If you try and move the pickup, it resists. Its extremely rigid, which is what you want. tone tranfer from the body.
Now, for the important part, which is what you really want to know-
how does it sound?
I noticed slightly more "Kerchunk" from the sound now. seems a little tighter and woodier on the low end. very subtle, but noticable to a trained ear. I think it is worthwhile tonewise. I would think the effect would be the same for one that is usually mounted from a ring.
You could put the ring back on after you hard-mount it. once the height is set, you wouldnt need to mess with it, so theres no reason to leave the ring off. the only difference would be that the pickup height
screw holes would be empty.
hope this helps. let me know if you try it, and how it sounds!