You mentioned that batteries are "too heavy and expensive". What's the energy density of hydraulic? You could spend your entire budget on high pressure control valves if you were really pushing the limit...
I've heard of hydraulic hybrid systems for city use in large trucks. I think they are high power but relatively low efficiency, and I really doubt they have enough capacity for long-term assist at high speed.
Of course the shortest route to 100mpg on the highway is aerodynamics and a very efficient small engine (see: aerocivic,
Edison2,
Avion).
If you really want to build a hybrid, by all means, tear into it! But there had better be a damn good reason to make it more complex than a small engine & good aero if you want to get it done and put enough miles on it to justify the work.