Bee-dubya-ell wrote:
"[...] the X21222 chord shape [...]"

This is a nice one. "2" on the sixth string is the fifth of the chord, so one can alternate it with the root on the fifth string when picking.

One can also use a barre on the upper fret and play a dom.7 b9 chord. The ninth is on the second string, so playing it one fret lower gives one the b9. One fret higher (without the barre) gives one a 7 #9. This is the chord (and I'm virtually certain the voicing) of the intro to Jimi Hendrix' "Foxy Lady". The fifth is on the first string, so playing it one fret below or above the "2" will give one a flatted and a sharped fifth, respectively.

A useful fact is that a diminished seventh chord can be considered to be a dom. 7 b9 chord without the root. So, C dim 7 == C Eb Gb Bbb == B 7 b9 without the B == B D# F# A C == D 7 b9 (no D) == D F# A C Eb == F 7 b9 (no F) == Ab 7 b9 (no Ab).

If one is ever looking for a way to modulate, diminished seventh chords provide (at least) one way of doing it.