The analog hole is not a logical explanation, because the analog hole exists regardless of whether you have a 5G or Classic iPod, and whether you use a 3rd party cable or a Apple brand cable/dock with authentication chip. Either way you're outputting analog video through component, s-video, or composite.
I suspect that they may possibly port rental playback to 5G iPods, but it's not a sure thing. The hardware and OS in the 2 is significantly different, so even if rental support is coming to 5G iPods it's not surprising its first available on the Classic models.
It would be nice to get confirmation from an official Apple source one way or the other, for 5G owners. There is a tradeoff involved for Apple too- they want to grow the rental market to generate momentum, but also want to sell new iPods. And I'm not sure there are a huge number of people that will replace their 5G iPod just for video rentals.
At this early point, I think it actually makes more business sense for Apple to support rentals on 5G iPods, as it would increase the user base and probably rental numbers as well. Steve loves saying those big numbers with lots of zeroes in his keynote.