Not sure if this is any help or not but I play a "D" over "B" this way. (it's tricky, and can't be done on a wide neck guitar, but if you train your wrist to be flexible it sounds good)

Regular D, and then bring your thumb over the 6th and 5th strings. Ignore the 6th string F# and start the chord from the "B".

It may seem impossible at first to get your thumb around the two bass strings without damping out the rest of the "D" chord, but it's not. It can take about five days worth of effort, but it'll give you such a flexible thumb and wrist that other simpler "thumb chords" will come almost instantly. You don't need big hands or long fingers...but you DO need a Gibson or Martin style width neck.