Is there any way to know ahead of time which paid apps are going free, or does Apples RSS feed actually contain data for apps that have just gone free "today" ?
There are a lot of Apps which provide a daily list of free Apps. These kind of companies base their business in agreements with the developers, who pay for their Apps to be featured on these App-of-the-Day lists.
So, the Company makes it's business, the Developer's App get notoriety and publicity, which means that on the long shot more people will download the App once it's paid again and the user gets to get free Apps daily. It's a win-win-win.
The best and easies way of knowing of paid Apps going free for limited time is using some of these Apps.
I use a a free app called Apps Gone Free which is updated daily with a list (likely not comprehensive) of apps that are free now or for a limited time.
No relation to the developer, I just use the app from time to time.
http://www.macappdeals.com/ also this site lists discounts on app prices.
Apple provide RSS feeds for their Mac App Store and App Store content. These feeds are used by other sites and services for finding free offerings, price changes, and new releases.
See http://rss.itunes.apple.com to create your own custom feed.
Apple's feeds are not able to directly answer your needs, but the additional filtering can be added by a wide range of other applications or services; or you could create your own with a scripting language.