Josh, this is kind of rough. I'm doing it quickly and just from memory to sort of give you an idea of what I'm talking about. It's the opening-figure in the I-position. If you've ever heard more than one recording of Doc (or someone else) playing it, or seen him on video or TV doing it, you know (like most trad-fingerstylists) it's never exactly played the same way twice. Anyway...
CODE
Hold this 076707 like this 0MIROP and play this (all picked with just a strong thumb down and the index up-picking the 1 and 2 strings)...
--7----------7--------------------------
-----0----------------(0)--------------
--------7--------------7------------------ (oh, a number in (), like (0) means it's okay
--------6--------------6------------------- or a matter of choice to sound that note too)
--7------------------------------------
------------------0--------------------
(Which you probably already do, or something very similar)
Now go to that A7dim (actually A7dimē9 maybe..? I'm no theory-Nazi. Anyway...) X05656 held XOIRMP (by lifting the middle finger off of the 5-string in the previous E7-chord and pulling it between your Index and Ring fingers to place it onto the 2-string as you slide the rest of that finger-shape a fret lower simultaneously which will now give you that X05656 (actually 005656 is the same chord) (but I digress.....) and play...
--6------------6-------------------------
-----5---(5)--(5)-----(5)----------------
----------6------------6-----------------
--5--------------------5-----------------
-----------------------------------------
-------------------0---------------------
Now sliding back up and moving that middle finger off the 2-string back onto the 5-string again into that starting A7 076707 chord once more play.....
--7---------------------
------------------------
------7-----------------
------6-----------------
--7---------------------
--0---------------------
and lift your Ring finger off of the 3-string 7th-fret and put it on the 2-string/7th-fret (other fingers starting to lift off the fret-board as you...
----------0---<(this open note giving you --------------------------
--7b8ub7--------time to quickly now move your hand ----2---0--------
----------------- down nearer the nut to play... -------------------
-------------------------------------------------------2---0--------
--------------------------------------------------------------------
--------------------------------------------------------------------
....or a similar figure that will have you into that next A IV-position at that end of the neck where you can do an A-figure.
NOTE: (added)
I'd like to say there is another common E7-shape that can be, and often is, used as the intro-chord. It's 0X6757, held OXMRIP. You play it and just omit hitting the 5-string as you alternate. You can then slide down a fret to that A7dim in a similar way, except now you'd leave the index-finger stationary on the 2-string and slide the other fingers down a fret to that 005656 also held OXMRIP (as was that E7), etc. Either way will work. Doc has used both (this last one maybe even more often than the example chord-shape I used above). Either works perfect and sound fine. I tend to use the first one (076707) because I play solo and I want as many notes of the lowest possible register in the mix or the lick can sound a bit too trebly (to my ears anyway). If you play with a band, you have a bass-man or rhythm guitar to take care of that. (The follow-up diminished chord also feels more comfotable to me too held as OOIRMP.) Just my general preference. As I said, either way works fine and Doc chords it both ways. Same with the little bent-note thing. There are several ways of doing it, each slightly different, as well as how you work into your IV-A figures when you move toward the nut. Never forget how ever-changing and improvisational these kinds of tunes are to the player as he is playing it.
Doc (and most people doing the tune) use a very chunky percussive semi-muted technique on the bass strings while playing the treble lines, similar to Merle Travis (who Doc patterned his own style after). So use the side of your right palm very near/on the bass-side of the bridge to get that same thunky-chunky percussive feel to it.