Not sure if you already started the process, but I started with the same situation as you ("old tank" rock that leached phosphates, may have been left to dry at some point). I was about to go the acid wash route, and was exchanging emails with someone on wetwebmedia.com who convinced me to try a less drastic path to hopefully eliminate the phosphate issue while also not sacrificing the heavy bacterial colonies that were probably around (thereby creating highly bioactive base rock). Here's what I've done so far...130# of rock...
First, like you, I gave the rocks a heavy (and I mean heavy) scrubbing, with 2 heavy duty plastic brushes (one large, one small for crevices). The rock seemed near-white when I started, but I couldn't believe how filthy the water was at the end of this.
Then, I started cooking the rocks. Brute trash can, circulation, heater (high 70s F), low salinity. I put GFO and Chemi pure in as well.
After one month, I repeated the scrubbing. Again, even though transferring from clear water, filthy/dusty water.
They're going on month 2. I will get a phosphate reactor for the GFO soon, and maybe even throw a skimmer on it. I see the nitrates rising as the month goes, so I know there is heavy bio activity going on. I also recently pulled a piece out to use in a quarantine tank for new fish, took some risk as I put 5 juvi fish in after a day or two (versus monitoring for a week to make sure it was cycled and not doing crazy things to the water), but it took the bio load with no problem and the fish/tank levels are doing great.
So I'm excited to see how this cooked rock comes out in my new tank build (still a few months away)...of course this path takes a ton of patience and occasionally a lot of work...but throwing out there what I tried...
Also, I think you'll kill all bacteria if you let it dry for more than a few days (in the sun especially), so I wouldn't do that if you're going to cook it. But, if you're going to acid wash it, then there's really no point in scrubbing/bleaching/drying because the acid kills all life and burns the outermost layer of rock off, so you could just jump straight to that and start from ground zero on it.
Good luck any way you do it. 
