For playing in E (or F or G with a full capo on the 1st or 3rd fret, and the "DADGAD capo" two frets further up), I can use either the DADGAD "drone D," fretting only the G string at the 2nd fret above the capo, or the full normal D chord - but still having the advantage of getting the low bass string open. For the sub-dominant, I use the "Tony Rice G" formation - index and middle fingers on the 5 and 6 strings, ring and pinky on the first two. Since all the uncapoed strings are fretted, the chord works just as well as if you had a full capo. You get an acceptable A7 by fretting just the fourth string, second fret, and the first string at what would be the third fret but is the fifth since that string isn't capoed...

Clear as mud?

Peter