It is gear driven. Actually, the more I think about it, there isn't MUCH difference between the HPOP from a 7.3 Diesel and an HPFP from the 3.5 ecoboost. The HPOP is driven by the timing gear set (the HPOP sits in the valley of the 7.3 at the front of the motor) and the HPFP is driven by one of the Cams in the OHC design of the 3.5. The only other differences are that the HPOP pressurizes engine oil to drive the hydraulically actuated HEUI DI fuel injectors and the HPFP pressurizes the fuel itself on the ecoboost to be injected by the electronically actuated DI injectors.
Provided that the HPFP can be improved in it's ability to provide addtional volume, while fitting into the same general footprint as the stock HPFP, it could prove to be a quite interesting option (though by no means cheap).