Ah, now that I'm back in the office, I see that the objects gotten in the selection set are AeccPoint objects, and the object I need to change is an AeccCogoPoint object.
I don't suppose there's a default function somewhere that does this conversion (AeccPoint <-> AeccCogoPoint)? It's simple enough to write one, but this seems like the sort of thing that should already exist. In fact, I'm sure it does; the question is, did Autocad expose it so that Lisp programmers can use it?
That seems to be a general failing in the Autocad APIs that I've been noticing. They've documented the object model relatively well, but if they allow Lisp programmers to access various helper routines, they've hidden that fact. For example, there should be a routine that will set the description for a newly-created cogo point based on the point settings (prompting the user to enter the description if necessary). I assume one exists; Autocad routines do this enough. But if Lisp programmers can access the routine, that fact isn't well-documented, so I had to write my own...