I have a feeling it's deliberate that it's set at an excessively long poll interval (and quite possibly disabled altogether) for free accounts (it used to be that way on LJ too, at least back in the day).

For the record, I can now confirm this: I upgraded to a paid account this morning and suddenly checkforupdates is behaving sensibly.

I still don't see the sense in the policy, and it would have been nice to have an error message rather than getting back an unexplained blank string (I did get a nice error message when I tried to use checkfriends, which is how I knew to rename to checkforupdates), but at least the cause of the issue isn't in doubt any more :-)