This could definitely be useful--but how much current can it supply (or how many milliwatt seconds in a burst once charged), and how much would it draw sitting idle?
my RC is woefully out of date, but won't the 15 uf be charging off the "leftovers" for pushing current to ground?
That depends on quite a few variables the ripple you are willing to accept.
Note that at any significant current, you "lose" 2x diode forward voltage (2x ~0.7V for standard Si-Diodes), so Schottky diodes might be worth it (2x 0.3-0.4V)
Some example values (Didn't test in reallife, this is simulated, assuming 100 Ohm output impedance of the arduino digital pins)
I found that increasing the 1uF doesn't do much, but drastically reducing it will decrease the performance quite a lot.
Concerning the duty cycle: 50% is the "best" in terms of highest efficiency and output, but reducing the duty cycle is a possible way to closed-loop-control the output voltage.
It's far from linear though and low duty cycles will cause high current peaks on the arduino digital pins, so on your own risk.