OK, From What I Know / AFAICT you would create the same
infrastructure layout as Trisquel but use the Trisquel repos as your
upstream not the Ubuntu ones. Your package helpers would tailor
specific packages to give you your distro's branding, desktop etc.
Hack makeiso.git to build your own install disk using them.
If you're going to run a .deb distro you will need to read package
maint-guide.
My guess is the main problem RMS will have been thinking of is the
fact that ordinary distro repos are littered with packages which have
GNU FSDG compliance issues. Using a GNU FSDG distro as upstream
saves you most of that effort, although I think to retain GNU FSDG
status yourself you would have to accept compliance bugs.
Leny