Most philosophers who are non-consequentialists are pluralists that think consequences are very important, so they can still use standard arguments to support the idea that reducing x-risk is important. A lot of non-consequentialism is about moral prohibitions rather than prescriptions, so I suspect most of it would have little to say about altruistic considerations. And of course a lot of it is loose, vague, and indeterminate, so it would be hard to draw out any comparative claims anyway.
Many people on Less Wrong believe reducing existential risk is one of the most important causes. Most arguments to this effect point out the horrible consequences: everyone now living would die (or face something even worse). The situation becomes even worse if we also consider future generations. Such an argument, as spelt out in Nick Bostrom's latest paper on the topic, for instance, should strike many consequentialists as persuading. But of course, not everyone's a consequentialist, and on other approaches it's far from obvious that existential risk should come out on top. Might it be worth to spend some more time investigating arguments for existential risk reduction that don't presuppose consequentialism? Of course, "non-consequentialism" is a very diverse category, and I'd be surprised if there were a single argument that covered all its members.