Brown out detector

Is the brown out detector enabled with the arduino boot loader? I would like to have it disabled, because I will rework my Arduino nano for 3.3V operation.

If it is enabled. At what voltage is it set to, and/or can I disable it?

(Even if I could disable it in the code, the bootloader needs to boot at 3.3V)

AFAIK the BOD is activated by fuses (BODLEVEL), and it is normally off. But it can be disabled by software as well (MCUCR),

IMO USB won't work on 3.3V, so that you can use a Mini as well, which already allows for 3.3V operation. More modifications may be required for a Nano, because e.g. the clock is/should be reduced to 8MHz on 3.3V operation, affecting the serial baudrate settings and all other timed operation (millis, PWM...). All that is automatically taken into account, depending on the IDE settings in programming a Mini.