Let me begin by saying that you matter to me. So, even if I am the only other person in the universe (besides you), you have achieved matterhood. Of course, I know that you matter to lots of other people as well.
I applaud your shifting the conversation from meaning to mattering, but I would argue that mattering is almost as slippery a concept. How do you decide whether something matters to you? You feel it, and that is about the only good yardstick. Yet you also feel it when you have done something "meaningful," don't you? But I think your point is that in both these cases, the mattering and the meaning have been defined internally, without reference to an outside being. That's an important way to look at this question, but one that does not satisfy everyone.
Your mention of mattering reminds me of something call "moral status." I have been recently mulling this over because I have been talking to the biologist Jack Szostak, at Harvard, who might be the first person to create life from scratch. He hasn't done it quite yet. The first cell will be very very simple, even simpler than a bacterium, but from there, it is inevitable that biologists will be able to create multicellular organisms from scratch, specialized cells, etc. Anyway, some bioethicists I have also talked to raise the question of whether human-made life forms will have "moral status." And by that, they mean, will they "matter" to us and to themselves? Will they be sufficiently "valuable," so that we have a moral obligation to treat them with respect and consideration? Does an entity deemed to have moral status need to be sentient? What about a computer that can learn by itself and communicate? Do things matter to it? Does it have moral status?
My intelligent ants certainly matter to themselves, and probably consider that they have lived meaningful lives as well. So, if we dispense with the requirement that meaning and mattering require an external agency, then there doesn't seem to be any problem. However, as you point out, most of us multi-cellular organisms masquerading as homo sapiens feel compelled to matter to the outside world, and have meaning for that world as well. Perhaps that's where we go astray.