PSIP can accommodate up to 16 days of guide data, if a station wanted to invest in the equipment and manpower to maintain their own guide. (And I don't believe it would cost that much, either.) In the past, I've seen KXAS/5 broadcast almost a week's worth of PSIP, so I know it can be done if a station wants to.
Sadly, though, it didn't last. It is a bit of a PITA to maintain, and not many devices use more than a day's worth of PSIP, if they use it at all; at this point I think it's basically just the DTVPal, CM-7000Pal, CM-7400, a few old CECBs, and offline DVR+ users. Even all together, that's still probably only a tiny fraction of a percent of a station's total viewers.
The way the PSIP protocol was designed precludes one station sending guide data for another, but there's no reason a different protocol couldn't be designed that did allow that. In fact, that's basically what the old TV Guide On-Screen service was. One station (typically a market's CBS affiliate) broadcast a weeks' worth of guide data for all the stations in the market. Too bad Rovi killed it.