i've just started riding my first hardtail in a while (mmmbop), my other bike has 7" of trail smoothing action.
I run:
2.35" tyres at about 30psi
Thomson inline seatpost
titanium saddle
Honestly can't see why you wouldn't be able to do an all day ride on an ally HT?
Yeah sure, if you sit down through all the rough stuff you're going to take a bit of a beating, but why would you?
Provided your bike fits and is set up properly, and you're not a little princess, you won't have any issues on all day rides.
The only thing i've noticed so far is a bit of a stiff neck/back if I've been constantly doing small-medium sized drops on the HT (e.g. 2ft to 4ft). I think that might be more down to my poor technique than anything else, since I'm so used to landing on a big travel full bounce.