Any IRIX app that uses TWAIN still cannot use the scanner unless you have an IRIX TWAIN driver for that scanner. Impresario won't do this for you.
Consider Photoshop. Photoshop supports TWAIN equally well on Macintosh or Windows (maybe even IRIX, who knows). You plug in a scanner that "supports TWAIN", nothing happens. Not until you install the TWAIN driver that knows how to talk to the scanner, that can send the correct commands to the scanner and get data back from it, then translate it into data structures that are meaningful to TWAIN, that Photoshop can use the scanner. Fine, now you have TWAIN scanning working in Photoshop.
Now, consider what happens when you buy a new scanner. Photoshop happily opens your TWAIN driver, for a scanner you no longer have. If it works at all, it sends incorrect commands to the new scanner, maybe gets data back from the scanner, but translates it incorrectly to TWAIN structures, because the data format off the scanner is different. Everything is weird, people are getting emotional. You install the TWAIN driver for the new scanner, which you should have done to begin with, Photoshop happily now opens the TWAIN interface to the _correct_ TWAIN driver, and it works normally, because it's the TWAIN _driver_ that is actually talking to the scanner. Photoshop didn't change. TWAIN didn't change. The TWAIN driver did.
You buy a scanner, it comes with a TWAIN driver, that knows how to talk to the scanner, get data from it, and expose it via a TWAIN interface. Now Photoshop (or any app that knows how to use TWAIN) can get data from your scanner. But you buy a different scanner, Photoshop can't talk to it, until you install the NEW TWAIN driver. An app uses TWAIN to talk to a device driver, which could theoretically be for anything. A scanner, a camera, a video tape unit, a thermal sensor array... any kind of source for image data. Then you don't have to rewrite your app for every possible image source. Just the TWAIN driver. But a Windows TWAIN driver isn't going to run on IRIX.