Terry... as Wyatt said, it already IS a reservoir... and what you're suggesting isn't anything new. I'm tryin' to remember who's already done that... someone has... and not only have others used the transaxle as their hydraulic reservoir, they've mounted hydraulic pumps on the aft end of the hydrostat, on the transaxle cover-plate of a narrow-frame... requires butchering out a bit of the underseat tool-box IIRC, and you need to find a suitable place to mount the valves you'd like to operate, and a good causeway to mount plumbing.
The thing to remember about fluid power systems, is that on fixed-displacement pumping systems, you're ALWAYS moving fluid... that means it's always returning, and in the process of making the trip, it gets warm, and upon return to the tank, it gets a little aereated. Now, the hydrostat unit flows a substantial amount, and if you do the math, and compare recommendations of most fluid-power design recommendations, you'll find that the Cub Cadet transaxle holds very little fluid compared to most design recommendations... typically they'll suggest one gallon of fluid reservoir capacity for every gallon-per-minute of pump flow capacity. This means the turnover-rate for fluid in a CC at full-speed is pretty high... so the fluid doesn't get much time to cool (convection through the axle housing), or diffuse aereation (bubbles) before getting drawn through the mill again. It's sufficient for the Cub Cadet, though, because the hydrostat unit isn't worked nearly as hard as it is in say... older skid-steer loaders where it also appeared. You'll know, though, if you start getting excessive aereation, you'll find that your pump and fluid temperature skyrockets (because you're compressing gasses, which applies to Combined Gas Law where Pressure=(Volume/Temperature).
But to make it simple, what you suggest is totally possible, and has been done. It also works, and if you keep that fluid level up, and don't work BOTH hydraulic systems too hard at the same time, it should work like a champ. If you push 'em hard, you'll find out what limited-reservoir is all about. FWIW, my Loader-Mutt's Johnson Workhorse uses one stanchion-arm as the hydraulic reservoir... it's capacity is 2.5 gallons, and the rest of the system takes a little over 1.5 gallons. The pump flows 6gpm at 1800rpm... and in typical operation, with the belt reduction I used, it turns around 1300rpm. Still movin' fluid kinda fast, but the reservoir (being tall and skinny) does a pretty good job of controlling aereation. we'll see how well it handles heat next summer.
.. email me info 
