I received this answer from a Enginer at Boise Cascade Wood Products.
It is ok to attach lightweight items like light boxes to either the
bottom flange or web of BCI joists. Generally, 8d nails are the best to
use for light loads such as yours. Drywall/wood screws less than 3/16"
in diameter are good too. The most important thing is to not use too big
a nail/screw as you could split the flange. Page 31 of our specifier
has a table showing the
closest allowable nail spacings.
Frank Powell, Jr. P.E.
Product Application Engineer