Apologies for the delayed response on this thread - I've been
travelling and at a conference with very little time to keep up with
email.
Thank you very much for your reply, Steve!
The current weekly images aren't using live-build, so the suggestions
you've had aren't going to help you if you want to reproduce what we
have. We're using live-wrapper instead.
The scripts we use are in the live-setup git repo:
https://salsa.debian.org/images-team/live-setup/
There's complication in there due to the way we build inside a VM, but
look at available/run-30livewrapper and available/live-customise.sh
for the core of how we build things.
I hadn't checked that repo yet.
I'll definitely try it and let you know.
Best,
Leandro