The short answer is: “Yes you can do that.”
We dynamically update our content using the API. What you’re going to find is that the current API (which is still fairly new) has some limitations on what types of items you can add/update. You can easily add Library items to a Layout using the API. However, it sounds like your system is similar to ours in that you won’t want to use Library items - you’ll likely want to use Embedded HTML or Webpage items.
There are a number of ways to do what you’re talking about. The first thing to understand is: What’s the format of the output from Nagios? Is it a graphic (JPG/GIF/etc.)? Is it HTML? XML?
To switch between general info and the Nagios feed I’d use a Campaign. One Layout has the general stuff and the other Layout has the Nagios stuff. If you’re not clear - Campaigns are basically an ordered run of specified Layouts. So this will accomplish your goal of switching between the two.
Hope this gives you some direction on this.