A friend pointed me towards tput(1), and I cooked up this solution:
#!/bin/sh
# ack-wrapper - use tput to try and detect whether the terminal is
# color-capable, and call ack-grep accordingly.
OPTION='--nocolor'
COLORS=$(tput colors 2> /dev/null)
if [ $? = 0 ] && [ $COLORS -gt 2 ]; then
OPTION=''
fi
exec ack-grep $OPTION "$@"
which works for me. It would be great if I had a way to integrate it into ack, though.