Recollection is that a couple of the major luthiery suppliers (e.g. Stu-Mac & International) were offering "graphite composite twin-bar truss rods" a whle back at a nice price and seemed to say they were shipping some; but I haven't updated my decade(s?) old catalogs so I don't know what's most popular now. I suspect that most significant makers stick to the more conventional parts with only a few variations in the constructions of the necks(?).

For those using grooves, for a time square and rectangular rods had some popularity (they don't have to be round), but I haven't heard of any people knowing they had one of those in an instrument.

John