I don’t know about a titanium pipe, but http://www.pyramid-plastics.co.uk do a rear hugger extender for the Duke 690 which I have fitted on my 701, though you need to put a couple of small bolts in to help the supplied glue stick it on.
I’m a bit of a Luddite so won’t get you a photo quickly. However, the extender is the right width as Duke 690 rear tyre is the same section as the 701’s; the extender is about 6″ (15.5 cm) long so gives that much extra coverage. It’s not as good as a full hugger ( after all it’s intended to extend the Duke’s hugger) but it’s much better than the complete lack of protection on the 701. And it looks integrated, conforms to the shape of 701 and only costs about £20. You can see a picture of it on Pyramid’s website.