Yes, it's POSSIBLE...
Whether anyone will code it or not is another issue, but you could get a good head start by looking at this page:
http://www.voip-info.org/wiki/view/Modified+Weather.agi+to+Work+with+Cepstral
If you use this for the weather forecast, be sure to use the "Alternative change" because it eliminates the unnatural pauses between each line of text. You have to have both the weather.agi and cepstral.pl files shown on that page. The big problem with the Cepstral voices is the price. You'd think they'd consider having at least one free, uncrippled version for non-commercial use. Even if it were only the Allison-8 kHz voice, that would give home users something to play with, and enough familiarity that they'd then later be comfortable using these voices on commercial systems should the opportunity arise (and it would then be much easier to get businesses to cough up the $30 per voice). Right now hardly anyone has any experience with using these voices, and I'll bet Cepstral loses a lot of potential sales as a result. It's not that the price is too high for use on a business system, it's just that hardly anyone knows how to add the code to make them work. People use what they are familiar with, and they gain familiarity by playing with their own, non-commercial systems, but few people will pay $30 just to play around with text-to-speech, when something like flite is free. Just my opinion.