Are capacitors floating power supplies?
To add to Barry's answer I'd also make these points:


  A floating driven power supply cannot send power through a path that does not directly connect its V+ and V- terminals, ... A chemical power supply, like most conventional batteries, cannot send power that doesn't connect its V+ and V- terminals,


Within lumped circuit analysis, no device can deliver power to any other device that isn't part of a circuit connected to two of its terminals.


  But a capacitor should be able to send power from either its V+ or V- terminals, right?


Current always flows in complete circuits. This means that whenever there is current flowing in one terminal of a capacitor, an equal current is flowing out of the other terminal. Because of this you can't say "the V+ terminal delivered power" or "the V- terminal delivered power". 

The capacitor delivers power to the rest of the circuit when current flows out of its more positively charged plate and in the more negatively charged plate. The capacitor absorbs power from the rest of the circuit when current flows in to the more positively charged plate and out of the more negatively charged plate. 

It doesn't matter which terminal is labeled "+" or "-", it matters which one is at a more positive potential and which way the current is flowing at the instant when you are measuring the power flow.