If it is necessary to do this, you can minimize damage as follows.
STEP 1: Remove all USB peripherals you can, and power the rest externally.
Place a single, POWERED USB hub between ONE USB port on the Pi and all USB peripherals. (Powered hubs have their own bricks. Unpowered hubs will make things worse. If you need more ports, DAISY CHAIN THE HUBS, do not use two ports on the Pi.)
RATIONALE: USB peripherals initially use up to 35mA per port, but can negotiate up to 500mA per port after that initial connection. If you used two powered hubs, you could still be drawing up to 70mA (though probably not). Using a single USB port connected to a powered hub guarantees your lowest possible host power draw.
Lower all clock rates-- CPU clock, GPU clock, memory clock, and the SPI clock in use for SD cards. (You can find the last one in the instructions for overclocking SD cards).
Put the clock rates back to normal after you get a proper supply. (You might even find you can overclock now, provided you use proper heatsinks and cooling.)
RATIONALE: The amount of power of power used per instruction is fairly constant. If you have less power available, you need to use less energy per second-- fewer instructions per second means less power used.
This step is easy, just tedious. Tweak one setting at a time, and test thoroughly. Start by knocking 25% off all the stock speeds. If possible, wire up an ammeter and MEASURE CURRENT DRAW BEFORE AND AFTER EACH CHANGE, it is easy to do and will be very useful for you to know.
STEP 3: Disable all GPIO equipment you can live without.
This includes individual features on each device, and this is often overlooked. Consider:
- Turning down the gain on transmitters and receivers
- Shutting off or dimming backlight displays
- Lowering display framerates
- Minimizing volumes on speakers
- Allowing longer acquisition times for GPS antennas
- Falling back to lower speed wireless standards
- Reducing ADC and DAC sampling rates
- Running motors one at a time, when possible (do not pan and tilt at the same time, etc.)
- Reduce signaling speeds (serial baud rates, etc.)
- Reduce optical gate/gray code wheel duty cycles
RATIONALE: Your payload is your purpose, but you may be able to meet your design goals in a round-robin fashion, using one device at a time, rather than just leaving all of them on all the time. This is the hardest step, but will also be the most rewarding for future designs.