Should I omit some of my qualifications in my application materials to "surprise" with during an interview?

No.

• How will you know exactly what's "just enough to pass the first selection round"? You won't. You can't.
• Don't "surprise" by withholding helpful information. I believe most people find it annoying and inconsiderate when someone deliberately withholds information that they need in order to do their job (which is what you are proposing to do to the selection committee).
• It's harder to change someone's opinion of you than it is to confirm it. Even if you manage to get an interview using this strategy, convincing the interviewer that the candidate he thought was mediocre is really exceptional is more difficult than making him think, "This student is just as exceptional in person as I thought he would be!" (Also see: confirmation bias)
• Presenting yourself badly on paper can lead the committee to think that you don't know how to present yourself well, or didn't bother to find out how, or didn't care enough to put effort into your application. These are not qualities that people look for in an applicant.

I fully agree with ff524's answer. As you still seem to think that your question outlines a good strategy, let me offer an additional point.

One purpose of having two rounds of selection is that the paper application focusses on different qualifications than the interview, and the committee wants both to be tested. So yes, by all means, do surprise the interviewers, but do that with points that couldn't be shown on paper due to their nature. Examples for these are:

• Give a convincing oral presentation about your results or plans.
• Show confidence even when asked critical questions.

The poster you mention may not be a good fit for the paper application anyway, but do not withhold any information about your research that would fit into the paper application format.

If you can show the poster at the interview, you would want the interviewers to be surprised by thinking "This guy has very nice research results, and he can present it in an innovative format." You don't want their evaluation to be "This guy cannot write clearly about his research, but ..." (nothing follows because you're not invited to the interview).

Absolutely not.

Even if you turned out to be a good candidate, I would reject your application on this basis alone. You're completely wasting my time by failing to be up-front about your qualifications when you know that I have a lot of work to do in prioritising applications and arranging interviews accordingly.

If I find out, after doing all of that work, that by hiring you I will be hiring someone who routinely withholds relevant information because he wants to "surprise" me, then I know I am only hurting myself in the long run. I would instead pick an individual who is capable of being up-front and honest — someone with whom I can work without constantly wondering what he's not yet bothered to tell me.