That's right. There's been minimal information on this topic. I've asked the folks at Our Families Count, who could not answer if such a thing would be possible. I've googled polyamory and census only to come up with a couple of people under the impression that being poly is not something that could be reported on the census. But once I received my census in the mail and saw the exact framing of the questions, it was right in front of me, clear as day: Poly families CAN be counted.
Poly Households Can Be Reported in the Census
There are limitations of course. Just as same-sex couples can only be reported if they are living together, the same is true for polyamorous families. This is likely a very very small fraction of non-monogamous people in general. Nonetheless, the census records the relationship between all household members and the head of household. If more than one household member has a romantic/sexual relationship with "Person #1," then they can each be listed as a married or unmarried partner. And that's exactly what I did for my household.
Normally, I like to know more about a subject like this before writing about it. Admittedly, this is the first census form I filled out, as I was still living with my parents the last time around. I don't even know if the questions were framed this way in the 2000 census. All I can say is that I hope this data gets used.
Plenty of organizations have made the point that the information about our families in the census can be a valuable tool in our fight for equality. This is no less true for families like mine. It's rather unfortunate that in the runnup to the census that our families were left out of the push to be counted and never even get a solid answer as to whether or not we could be counted. When the data is released and it's time for the number crunchers to do their thing, I hope we don't get left out yet again.