You can configure motion to run external commands on various events in your motion.conf file - see http://www.lavrsen.dk/foswiki/bin/view/ ... l_Commands
For BrightPi, you could create shell scripts that run the various i2c commands in the BrightPi demos to control illumination, and then set motion to run these on various events. Or you could write short Python scripts instead using the smbus package to send i2c data to control the BrightPi.
However be aware that motion will have to be detectable with the BrightPi turned off for this to work. Also, turning the BrightPi on will affect the image data motion uses for subsequent detection, but that may not be an issue (as motion has been detected).
If you turn off the BrightPi when the motion event ends, the change in light level might then be treated as a new motion detection event, switching the LEDs back on again. There are options in motion to try to deal with this - you'll need to experiment (see http://www.lavrsen.dk/foswiki/bin/view/ ... ightswitch
If you're using a Pi NOIR camera, you could keep the BrightPi IR LEDs on permanently and turn on the while LEDs when motion is detected. That's something I plan to try myself when I get a NOIR camera that actually works.