You can use a DC to DC circuit, but bear in mind.
Taking Ohms law into consideration though, it will cut your tested time in half because it needs more amps to produce a higher voltage.
So this means that if the Pi requires 500mA, then the DC-DC on the battery converter actually needs 1000mA from the batteries, more or less. Since 2.4 to 5v is roughly half.
The next problem with powering
3v3 is that you need it be
3v3 constantly, because once the batteries get to about
3v it will start to brown out and rebooting your Pi, cutting your actual time into something more like 1/4 of what you'd expect.
The voltage regulator on the first Pi's was a NCP1117-3v3 which
claims it can take up to 20v maximum input, I wouldn't try this
because allot of changes are required to make that work properly. It also claims that it can handle a 1.2V maximum drop out voltage, but that comes with a price too, read further.
It is true that the input 5v is generally used for USB and HDMI... but that was not a design choice by Pi, but a world wide standard that is used in electrical designs. The 3v3 regulator works best when its powered by
5volts, not by 3v3 (unless its constant which a battery wont provide)
Taking that in to consideration, it would be best to connect 4*1.2v (rechargeable batteries) and connect it to 5v rail (USB), but you shouldn't use HDMI or USB devices to help prolong the battery life.
The 3v3 regulator will work at its best then, giving you a much better result in the areas of 75-85% efficiency.
If you connect 3 x 1,2 you will only yield about 40-50% efficiency on the 3v3.
This is obviously taking into account you are using 2xAA batteries
mah - If on the other hand, you batteries with 10 times
the capacity, you could use 3.6volts because the batteries will be
able to sustain an acceptable voltage for much longer... but still
only towards the high 50% efficiency.
Once the voltage starts to drop, more power is required by the regulator to sustain the output voltage (ohm law again), and plotting a graph you would produce a parabolic curve over time, with an extremely sharp downward trend under 3.3v and a complete cut out near 3v.
If you interested in a true battery operated Pi that wont cause you headaches, look at my other answer. The initial investment surpasses the return value, since my Pi can run for many days if you adjust accordingly. You dont need to buy all the equipment, you can replace my solution with 8xAA reachable batteries instead, but investing in that will only return a fraction of power up time because of smaller capacities.