Does it have to be carbon? There's definitely slack and low 29ers made out of titanium if you're after lightness or steel if you're on a budget.
I think most likely those who are after slackness in a HT are going to want some compliance in the frame which is more easily done with the springier metals. Those after a light racing frame would rather have the stiffness, which is why most (all?) of the carbon hardtails are going to have racing geometry.
Hopefully somebody else knows of one.
'12 Santa Cruz Superlight 29 | '12 Santa Cruz Butcher | '06 Specialized Allez Comp | '81 Schwinn Converted Fixie
Trek Superfly 9.6-8 have 69.6 with a 100mm fork. That would go slacker with a 120. CS is 435. BB is 12.24 at 100mm travel. It has exceptional compliance.
Pivot Les is 69.5, 435 and 12.1.
Titus Fireline Evo in ti is 68.9, 434 and 12.1 with a 120 at ride height.