XNU is open source and I have personally used custom kernels, but if it got to that point I
definitely don't think it would be worthwhile for Little Snitch to maintain their kernel extension.
I truly don't think it would get to that point though. And even if it does, that day could be years away. We're talking about maintaining an existing product, not starting a new one from scratch.
IMO, the more pertinent question is whether it's worth asking customers to disable SIP. Up until now, commercial Mac software—even software targeting advanced users—has seemingly wanted to avoid that at all costs, whether it's Flavours discontinuing their theming software or nVidia discontinuing their web drivers†.
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† Note that I'm continually suspicious we don't have the whole story here, but the commonly-cited narrative is that Apple won't sign nVidia's drivers.