Yes, this works.
The rules say that when you make “an attack roll” and succeed, you “hit” and then you deal damage. Tripping says you make a “touch attack,” which is an attack, and then provides an exception to the “deal damage” rule, namely making a trip attempt and then the target becoming prone if you succeed on that. But nothing says it isn’t a “hit.”
The other part of the condition is that you are “using Power Attack,” but Power Attack explicitly applies to all attacks until your next turn, and there are no limitations on using it (even light weapons, which cannot benefit from Power Attack, can still use it—they just take a penalty with no damage bonus).1 So that’s no problem, and indeed, using Power Attack with a trip attack is pretty common if you are also using Improved Trip and expecting a real follow-up attack for damage.
So you are “using Power Attack” and you “hit” your target, so yes, you trigger Knockback.
Since Knockback specifies that the bull rush attack happens as a free action, I would rule that it occurs after the trip is complete—and so you are, assuming you succeed, bull rushing a prone target. The rules don’t explicitly mention the prone condition, but I for one would definitely give the target the +4 bonus for being “exceptionally stable,” as shoving someone laying down is much harder than shoving someone standing up.
But one of the exceptions here—arguably, anyway—is one you may be very interested in, based on your other questions. Eldritch blast is a weapon-like spell effect, as defined by Complete Arcane, and while Complete Arcane lists a number of core feats that work with weapon-like spell effects, Power Attack is not one of them. This implies that Power Attack cannot be used with weapon-like spell effects. And eldritch glaive allows you to use eldritch blast “as if” it were a glaive, but it doesn’t actually make it into a glaive, so it’s still a weapon-like spell effect, not a weapon.
The counterargument is that Complete Arcane doesn’t have to mention Power Attack, because it already is quite explicit about “all melee attack rolls” and “all melee damage rolls.” And even if you cannot apply Power Attack to eldritch glaive itself, you could still choose to “use Power Attack” prior to making your attacks for the round, and thus be in a state of “using Power Attack” which should work for Knockback anyway. Alternatively, you can also try to argue that “as if” is strong enough to make the eldritch glaive count as a glaive in pretty much all ways—this is a more dubious argument, however.