I am in the throes of learning open tunings as well. There was a really helpful tip given by Paul Reisler in an article in Acoustic Guitar Magazine. He notes that if you think about the open forms in terms of 1st, 3rd, and 5th of each key (say D, F#, A in open D), then you can relate the tunings of open D, open G, and open C -- the 3rd of the chord is on the open third string in the D, on the open second string in the G, and the open first in the C). And the 1st and the 5ths can be seen to shift down the guitar similarly. You can move a chord fingering pattern learned in one of them up or down (until you run out of guitar) as long as you shift into the 1,3,5 of the new key when you change tunings. (So a G in a D tuning looks like a C in a G tuning one string further towards the bottom, and so on). It doesn't solve everything, but it helps a bit in the myriad of chords.
Joni Mitchell uses an open D tuned down to C (CGCEGC) on one of her best songs - Amelia. I can't decide if I like that C better than the open C discussed above. And there is the "low C" -- standard tuning with the lowest E as a C and the A as a G. I am just trying that one thanks to my teacher who is currently eating in a McDonalds somewhere in Perth.

yours, Peter T.