Some strategies for turning this to your advantage, if your ultimate goal is being involved in the design, not the evaluation:
- Make it a test that's just Pass/Fail. Don't allow room for wiggling on the results, or you wasted your time getting them data they don't want to hear. Agree up front on what this thing is supposed to support, scenarios they think it has to handle, and be firm on delivering the news afterwards. It's too easy to get wishy-washy about informal usability testing, and leave room for argument, otherwise.
- Create alternate designs for use in the usability test. One of the best ways to educate the organization about the value of design is to DO IT. And to turn the test into an evaluation of multiple designs will help build your credibility and turn post-hoc evaluation into formative, and more useful, usability testing. Throw in some mockups at the end, or at the start, and ask for comments on those in comparison to the product that so clearly has problems, from your perspective.
- Write the report before the test. No, not as unethical as it sounds - you won't deliver it, but you're checking on your skills to predict what's going to be hard to use. So, you got all worked up about how this thing is hard to use, for obvious reasons; now check to see if you were right! Better yet, have more than one person on your team do their own reports, privately, before the test, and seal them up. Afterwards, see who was the best at predicting the disaster. Chances are, the users will fail in ways never imagined (thanks to Kevin Berni for this line), and you'll be a little less cocky next time this situation comes up. But if you get it all right, you're building your case and confidence for pushing back on this kind of request next time around.
- Ask others in the org to predict what might cause the users problems in the current design before the test. I used this tactic once when a QA organization felt the way I did about the usability of the product we were testing. I gave an award to the person with the most accurate prediction of user difficulty after the analysis. You want everyone in the company to know who has good insights into design and usability, right? People with good instincts need to be making the judgment calls, in the future. And you need to be able to illustrate that not everyone's opinion is equally valuable when it comes to making design decisions. Design insight requires talent and skill. This contest before the usability test helps identify talent-- skill development can come later.