I wouldn't have thought it'd be too hard though, as the system could simply flag an article with 'Show HN' in the title when it's added. And then /show could search for those articles with the flag.
It seems like the "ask" link is just a collection of threads that do not point to an external URI, though, since not all of the threads there even have "Ask HN" in the title. Indeed many of them are show threads.
Not that I disagree that it would be difficult to do this, I'm just pointing out that I don't think this is how it's currently done anyway.