Resistance of electronic circuits
Yes you can - although "resistance" isn't quite the right word for a complex load such as an electronic piano.

Rearranging your formula slightly

$$ V_{ripple} = \frac {1 }{ 2 f R_L C} V_{peak} = \frac {1 }{ 2 f C}\frac {V_{peak}}{R_L} = \frac {1 }{ 2 f C}I_{peak} $$

since I = V/R.

Since you have a steady current of 0.5 A you can plug that into the formula. You'll get the same result but without the intermediate calculation of the "resistance" of the piano.

Note that you should use the calculated value as a minimum. Doubling it won't do any harm other than raising the average voltage a little and a slight increase in power dissipated in the regulator.