I would hesitate to add anything to a nailing surface other than wood. You would best advised to add 1/2" plywood, or if that is too thick try just 1/4" underlay. If the 3/4" plywood is sound and sturdy, all you really need is the thin plywood to smooth out the hollows. But if there are large hollows and the hardwood has to span a gap of more than 4-6" you might consider installing the thicker 1/2" plywood.
I'm sorry but there are no quick fix compounds that will hold a flooring nail. Any epoxy leveling compound spread over the whole floor will be more expensive than the plywood. And these can take quite a while to cure, and cause quite a toxic stink in the house. These not used in the professional wood floor industry (except for concrete), only by uninformed amateurs. And just look at the amateur mess you had to clean up.
But you know, as long as the gaps are not too big you can, in most cases lay the hardwood over this existing subfloor just the way it is. Just make sure that you don't put a hardwood floor joint on these gaps in the subfloor, and be sure and nail the hardwood floor every 6-8", crossing (at right angles) to the joists.