I built up a set of XM481’s on 240s about a year ago and had the same concerns, I have never been a fan of Alu nipples. But after having the same issues as you regards finding the brass ones I built them up and they have been absolutely fine. From what I understand the nipples are coated on the inside with what is essentially thread lock. This works as a corrosion barrier as well as stopped the nipples backing off.
By way of disclaimer however, my bike lives in the house so it never sits in a cold wet shed/garage which may well be mitigating any issues.
Also, road nike has DT wheels with alu squorx nipples and they have been fine too…
Edit: I was also careful to smear the PHR washers with a little grease while building the wheels too.
I’ve had aluminium nips on my Kona since 1996 and they are fine. Make sure you/your wheel builder remember to grease the spokes before adding the nipple (should be done regardless of nipple type) which keeps the steel and alu separated. Keep the bike away from salty water that may act as an electrolyte and start galvanic corrosion. They won’t seize up if kept dry and if the metals are separated. If that’s not going to work for you then buy brass but still grease the spokes!
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