Let's Do the Numbers
First of all, much thanks to Paradox for posting that awesome chart.
Bleach
Over the previous four weeks Bleach averaged 902,750 viewers. Bleach's Toonami rating for the night was 1,010,000 viewers, a 12% increase over the four week average. Pretty good I'd say. It should be noted that Bleach hit 1,003,000 the third Saturday of the month, and the Toonami total doesn't top that by much. However, the Toonami total is significantly higher than the couple of weeks in March when Bleach was in the 800,000s. The four week average helps to smooth out the variations.
Dragonball Z
Dragonball Z held onto 99%, or essentially all, of Bleach's viewers. That is amazing and a big bump from Fullmetal's four week average of 80% retention. A lot of it was probably the curiosity factor of what would happen next, but just the fact that that many people were interested is significant in itself.
Gundam Wing
Gundam Wing retained 81% of DBZ's viewers (and 80% of Bleach's viewers). A bigger drop than DBZ's but still impressive considering that Durarara is on average only holding onto 63% of Bleach's viewers at this point. Gundam Wing's 806,000 total viewers is higher than anything else non-Bleach on Adult Swim Action in March (outside of a a couple Fullmetal episodes) and close to Bleach's ratings for a couple of Saturdays in March.
Tenchi Muyo!
At this point some comparisons get trickier since time slots no longer match up, but we can still track audience retention, and I may run some other numbers anyway. Tenchi Muyo! retained 96% of Gundam Wing's viewers (and 77% of Bleach's viewers). No big dropoff here, especially compared to Gundam Wing's drop. Apparently the rather confusing episode didn't chase away viewers. (I can't say I could follow what was going on all that well, but it was cool anyway). Also, the total viewers (769,000) still beats most everything on ASA in March (see Gundam Wing).
Outlaw Star
Outlaw Star retained 102% of Tenchi Muyo!'s viewers (and 78% of Bleach's viewers). It should be noted before I get too much further that retention of Bleach's viewers assumes viewers aren't being added and instead there is just dropoff throughout the night. Thus it isn't completely accurate, just look for instance at Outlaw Star's increase over Tenchi Muyo! However, considering this is the overnight hours, there is a good chance it is accurate more often than not (plus it just gives me another way to generate numbers).
Anyway, this is more than a fantastic hold for Outlaw Star. It was the only show of the event to gain in viewers! The total viewers (786,000) also once again still beats most everything else on ASA in March (except for those episodes mentioned in the Gundam Wing paragraph).
Big O
Big O retained 98% of Outlaw Star's viewers. Basically, most everyone who made it to Gundam Wing stuck around through the next couple hours, despite the time being past 3 AM on the east coast. And the total viewers (769,000) still beat most everything else on Adult Swim in March outside of Bleach and those 2 Fullmetal episodes.
Yu Yu Hakusho
Yu Yu Hakusho's drop is a little bigger, but it still holds onto 92% of Big O's viewers at this late hour (and 70% of Bleach's viewers). This time slot is usually only holding on to an average of 47% of Bleach's viewers by this point. Yu Yu Hakusho's total viewers of 706,000 are significantly higher than that time slot through most of March. The one exception is March 10th when ratings were abnormally high (compared to the other weeks in March) throughout the night.
Blue Sub No. 6
Despite a lot less name recognition and probably only being well remembered by the more diehard Toonami fans, Blue Sub No. 6 still retained 84% of Yu Yu Hakusho's viewers (and 59% of Bleach's viewers).
Trigun
Trigun kept more viewers around with a retention of 94% of Blue Sub No. 6's viewers (and a retention of 56% of viewers since Bleach.
Astro Boy
Despite heading into black and white territory, Astro Boy still retained 89% of Trigun's viewers (and 50% of Bleach's viewers). Usually by this point in the night the audience is only 39% of what it was during Bleach.
Gigantor
Gigantor held onto 82% of Astro Boy's viewers, the second biggest percentage drop of the night. However, Gigantor's 414,000 viewers (ignoring the overperforming March 10th lineup) still beat about half of the Adult Swim Action lineup in March. That's a lot of Toonami fans sticking around to 6 AM!
I'd say the biggest numbers here are DBZ's retention of nearly all of Bleach's audience, and the hours of Gundam Wing through Big O (or we could even say through Yu Yu Hakusho) where there was not only a high number of viewers, but very low audience erosion from show to show. The biggest drop in viewers was actually Gundam Wing, but the total viewers were still high, and perhaps the initial curiosity factor had worn off for some viewers by then.
One other note from looking at Paradox's chart. While Adult Swim Action's ratings may be low compared to other nights of the week, considering how much lower Family Guy's ratings on Saturday also are compared to the rest of the week, a lot of it seems to be the day itself and not Adult Swim Action. In fact, taking the time slot into account, Adult Swim Action does reasonably well, and did much more so on April 1st. Overall, I'd say Saturday works pretty well. If ASA was moved to another night instead of Saturday, it would just face more competition (That's not to say I wouldn't mind seeing an actual expansion of ASA though).
With the right mix of shows and attention, Toonami on Saturday nights would be a good chance to build something in a time slot that probably wouldn't have many viewers otherwise.