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The Shimano spokes are proprietary with flattened spoke heads, but I've been playing with some old C35 hubs and some straight pull CX-Ray spokes and they seem to fit fine. I can see that the flattened heads would be useful when you're building them in a production environment, as it'll stop the spokes twisting when you're building them up, but at the moment I can't see any reason why CX-Ray straight pull spokes shouldn't work. Haven't got round to actually using them in a new wheelset though, although I do have the rims I need to reuse them.
Found this post on Weight Weenies which is the only reference I can find to someone rebuilding these hubs. Doesn't sound ideal.