Mark,
As one who has spent a considerable amount of time studying Doc's fingerpicking techniques, the "muting" is an integral part of his sound. The mistake most people make when trying to emulate this, is dampening too much with the palm. This was pointed out to me at a lesson a while ago by Rick. Only a very slight amount of pressure should be applied to the 6th, 5th, and 4th strings...so that their tone is still heard clearly, and that the remaining strings are allowed to ring out. Muting is a technique (much like Clapton's "slow-hand vibrato") that can take months, or even a year to really perfect. You can experiment adjusting your hand angle when muting, so that thumb index, and middle are able to comfortably play...but you will have to fool around with it. I'd suggest 15-20 minutes on this a day (in addition to any other practising you're doing)...and you should have it down. Don't get frustrated if you don't in a week. It took Doc years to achieve that tone and style.

When Doc flatpicks, he rests his baby finger on the pickguard as well, which he claims is a "depth gage" for him, as a compensation for his blindness. Many flatpickers don't have an anchored pinky, as they like to use their entire arm. Most of what Doc does is generated by his wrist and lower fore arm.

What you refer to re: RGD I think of as "two fingered flat-picking". I've been religiously working on this technique for the past 4 or 5 months (as it was the one area of my fingerpicking technique that was weak.) It is something that takes a lot of practise and months to finesse. It is an invaluable technique!!!! In addition to allowing you to insert single string runs interspersed with your fingerpicking, it can also substitue for flat picking as well.

In order to practise this method, I'd suggest starting with basic scales in all the primary major keys...and doing it very slowly. (You might want to use a fingerpick and thumbpick as it will give you a sharper, stronger attack. I use them, but have nails long enough that a lot of the time, I can get by with just a thumbpick and nothing on my index finger.) Then start figuring out some runs, from your favourite bluegrass artists and learning to play those runs. Practise them very slowly so that they are true and clear, and then very gradually start increasing the speed. Since I don't have anyone to play with on a regular basis these days, what I've done is create a bunch of sequences with chords, bass, and light percussion, on a computer sequencing program, and have it midi'd to a polyphonic synth. I have tons of them saved in different keys (ie: Arkansas Traveler, Bromberg's "Foggy Road to Milledgeville" and a bunch of others.) I start the tempos at around 120, and play against these sequences for 15 minutes to get loose...then I gradually increase them so that they're up to around 210 or 220, and then it gets fun! I had a friend over here recently who is a very good fingerpicker herself, and I told her to close her eyes and just listen and to tell me if I was fingerpicking or flatpicking. Then I played along with Arkansas Traveller at about 180 beats per minute. She couldn't tell the difference, eventhough I was doing it 2 fingered style. I do believe however that flatpicking gives you an even stronger attack and more sustain...so this is something I will want to perfect in time. But for now, the two fingered method is invaluable. Go for it and don't get discouraged.

Ted, Robert Johnson was a master at getting tremolo effects while keeping his thumb going. Works best in blues and country blues that are in Drop D.
Cheers,
MK