I'd like to be able to build an install image using ImageBuilder that has a custom script in /etc/rc.local. OpenWRT 18.06 seems to have a bug on one of the devices I'm using, so I'm working around it by putting this in /etc/rc.local:
(sleep 30; /etc/init.d/dnsmasq restart) &
I'd like to build an image that has that line already in it. Is that possible using ImageBuilder?
Yes.
Follow the advice about custom files in wiki. you need to use FILES= variable in the imagebuilder command line to include a ready /etc/rc.local file into the build.