If we're just going for HP, don't forget the forsaker.
Add all the +10 bonuses into CON. (some pepople claim that you can only add five into con because its an "inherent" bonus, but that's a tad silly.)
That gives con of (assuming starting 20):
20+10+5 (from every-five level increases) ... for a con of 35.
Now, since you've forsook magic, you may as well forsake all material posessions. Get the Vow of Poverty feat from Book of Exhalted Deeds, and can add +8. That gives a con of of 43.
At 20th level, that's 400 HP just from con. Add in the 20d12, assume average die rolls, and you get an average con of:
400+12+123.5, or 535.5 HP.
Add in the toughness feats and it just gets stupid. Will it break 600 on average?
::edit:: Apparently its 65 HP from the Toughness feats over 20 levels ... So yes, it'll break 600. Just barely. And if you max on all those dice (as if, but just for record keeping), you'd have about 710 HP. Isn't that stupid-high?
And, i do believe, its all legal.
Second Edit: I forgot all about rage. With 9 levels of barb, that's only a +4 to con. Which means um ... another 20 HP. So make that 730 HP w/maxed hit dice. And 505 before hit dice.
Is that all right? Who thinks that's incredibly cheesy?