Well I have researched extensively on the subject, so I would have to fish around to find references. But basically hydro and oxy function very similiar, in that they are both metabolized by cyp2d6 into they're morphine based parents, that are far more potent than the -codone versions. But both of them convert into very small amounts, which is why oxycodone bears only a passing resemblance to oxymorphone and hydrocodone is very much different than hydromorphone. But what separates them, to the extent of my knowledge and, perhaps more importantly, experience taking them with/without all the various inhibitors, is that oxy is actually a potent agonist on it's own, allowing it to act independently of it's metabolite, whereas hydrocodone, though definitely active by itself, is much less potent, making the jump to hydromorphone more important. It really varies though, based on individual metabolism, tolerance, etc etc etc.
It is a very interesting subject to me how drugs work in general( in case ya hadn't figurd that out

) and this one really interests me, but overall, neither may be "true" prodrugs, since they both have they're own potential, but ultimately I believe hydrocodone does rely on hydromorphone for it's effects, unlike oxy, which is fine on it's own!!!
(Edit to say that the hydromorphone is small in any case, and we all know hydrocodone is one of the weakest opoids, so whether it gets it's effects from the small amount of hydromorphone, or it's own limited effects, it is still lacking in potency. Also, I believe 10-15% of the dose is converted to hydromorphone. that would make 30mg h-codone convert to 2-4mg hydromorphone, which by itself would be very weak orally due to low oral BA, but since it is metabolized in-vivo, this is less of an issue, giving you much better effects)