Today's family is a footing zone of influence object. We've tried a couple things to accomplish this before, but it seems that creating a simple face-based generic model with shape handles is the most flexible.
(You should also check out Myers' description of how useful these sorts of things can be over on his VDC blog for what he's doing out in Boston).
So, without futher ado, meet the family:
Once loaded up, snap it to the bottom of what ever footing needs it:
Adjust the shape handles to match the size of the footing:
And then check the type and instance parameters to make sure it is set correctly to match the conditions being represented (ie: it goes deep enough into the ground so you know where you can and can't run things near footings).
And here's a shot of the familiy in the wild (selected in blue for clarity - you can see his siblings in "clearance yellow" next to him):