If phantom power is 48v 10mA DC and you inverted it to AC and the back to DC, how many milliamps could you squeeze out of it at 9v? With the conversion losses, would it even be possible to get something like 50mA?
You need to think in terms of power. Phantom power is 48V through a 3,4k resistor, so that's 0,68W. The full power output is basically a short. That doesn't help you much, because then you have no voltage to run anything. You get the maximum power transfer (https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem), when your load is the equivalent of a 3,4k resistor. That means, you halve the available voltage, and you have 24V through a 3,4k resistor to work with, which is 170mW. If you assume 100% efficiency when converting it to 9V, you can achieve 170mW/9V = 18mA as a maximum. So in reality it will be only 80-90% of that, depending on your DC-DC switcher.