I think video of a child working/thinking about above-level work, plus a copy of that written work, makes a very persuasive portfolio submission. I'm convinced that most of the time, if you see an HG+ child at work, you know it.

Also, I'd recommend shooting for capturing achievement that's more than 2-3 grades above level if possible. I think gratified3 is right that they're asking to see outlier status, so further out on the tail of the Bell Curve is better than closer to the middle. Pick the stuff your child does that makes you think "That can't be normal, can it?" and use that for the portfolio.

Personally, I'd start with the portfolio materials if you have anything that would work. If that doesn't cut it, you can always test and resubmit.