Can I configure the relative brightness level that auto-adjust will set? I enjoy the Mac OS X brightness auto-adjustment on my MacBook Air, but I would usually prefer the screen dimmer than the level it decides on. This would also conserve some charge.

I don’t suppose there is a way to bump it down a few notches? (I have searched for this and found nothing, so I assume the answer is no. But it is difficult to pick keywords that will produce relevant rather than almost-relevant results so there is a slim chance that I am overlooking it, and I shall remain hopeful yet.)

This would be in 10.7.4, in case that is relevant.
You can use Shady.
The way it works is that it...

...pull(s) a virtual shade over your screen ... dimming the display to a more comfortable level.
Shady will save your shade level and restore it next time you launch the app.

As it's just a virtual (but effective) dimming method, I doubt that this saves much energy, but I cannot confirm this as LED displays should consume less power if displaying darker colors.