I dout there would be much waste, the arch wouldn't have to be continuos per ply, just dont pile up the joints in line with each other, the glue is stronger then the wood anyhow. If I were to do it I'd likely do it in 1/3 - 1/2 - 1/3 . . . plys put em in a vac bag, and wala. If you wanted to over kill the joints, scarf them.
Edit: As Frank mentioned the Tj's (the osb capped by dim lumber) can and do span further and hold more weight on the longer span then their relitively sized solid wood joists.
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