I called Longleaf, but he was not real optimistic that he'd be able to get the US importer to add that to the accessories they carry. He thought I should just order it from the Netherlands, but it is a major pain in the neck trying to get shops in the Netherlands to ship to the US; I've tried it before
So I opted for a UK distributor, instead:
And I figured as long as I was doing this, I'd get both sizes for front and back seats, since there's just no telling how that's going to go. In a way, I feel a little silly, because I feel like I should be able to reuse the equivalent sleeping-bag-with-holes-in-it from the stroller, but the harnesses are different and, more importantly, the legs need to be separated.
Here's hoping it shows up.
I don't think there's an American English word for whatever the hell this is, and yet here I am about to own two of them. It does not seem right.