On projects that I have worked on in the last few years hydrovac has been used many times for excavation for installing piles, and I don't recall ever seeing different friction values being presented by our geotechnical consultants. Having said that, in most cases hydrovac was only used for starting piles (i.e. to get past potential uncharted underground utilities in refineries, petrochemical plants, etc.), and then once past this danger zone (which in most cases is close to the same length as the dead zone on the pile - where we don't count on any friction capacity anyway), the contractor continued drilling the pile with a conventional drilling rig.
There are strong caveats for drilling deeper with hydrovac, not the least of which is that depending on deeper soil layers/materials your pile might be a whole lot larger in diameter than you expect, even to the point where you could fail adjacent structures. The deeper you go, the harder it is to see what is actually happening. And in some circumstances even shallow excavations have resulted in undesirable effects (such as cave-ins) due to a sandy-gravelly layer washing away, rather than cutting a relatively nice clean hole in some clay/clay till.
Discussion with a colleague reveals that while he has never seen a different value for friction capacity, he has seen higher values suggested by the geotech for frost jacking effects.