What's an efficient way to get below the standard 1.25V on buck converters?
You can use a fast op-amp to amplify by 2 and feed it with the output voltage (say 1.25V). The output from the op-amp feeds the reference input and if you do this correctly, the switcher will force the pulse width down until there is 1.25V on its reference pin. This means the output MUST be at 0.625 volts.

In other words you have conned the switcher by using a gain stage. There can be stability issues so use a fast (possibly 10MHz) op-amp like the AD8605 and be prepared to add some filtering capacitors.

You can also do this by adding a DC offset into ref signal. So, take the output voltage and pull it up with say a resistor and diode. This raises it by ~0.7V and will force the switcher into producing an output that is 0.7V lower on the output. No gain involved here so stability probably won't be an issue.