General CL question: get-setf-expansion

I'm writing a macro I've wanted for a while, which looks like (defaulting <place> <value-form>).  This:

evaluates <place> for a value or values
if it's nil it then evaluates (setf <place> <value-form>)
and then returns the value or values of <place> again
There are various options for fancier cases.

To do this right, it needs not to evaluate subforms of <place> more than once, so, for instance, (defaulting (aref x (incf i)) 0) can do what it should.

get-setf-expansion provides what I think I need to do this.

The question is: does anything usable as a place to setf have to have a setf expansion, or is it allowed for things just to fall back to calling some presumed (setf x) function?  I don't think it should be because if it was it would make writing a macro like this much harder, but I can't work out from the spec.  Both LW and SBCL are empirically fine.

Sub question: someone must have done this sort of thing, and I'm sure I can remember seeing things like it.  I can't find one now though: is there some available equivalent?  (Please don't tell me this is already in CL: I will die of shame.)

--tim