Create something similar to this
I've wanted to do this, but haven't really developed the expertise as of yet but to create an config file similar to http://www.easyvmx.com/ creates vmx files. It could create a shell script so that you select your distro and then check (or uncheck in some cases) the programs you want and based on your selection it would install or remove the programs in the selected distro.
Edit: It may not be paticular programs but maybe groups of programs (e.g. Remove Office Utilities, the 500 bash shells, etc.) then the install programs could be leading programs that would most commonly be used in CarPCs (e.g. GPS, Multimedia, Power Saving Apps, etc.)
A really really crazy design would be allowing the user to create a viable frontend with just a WYSIWYG editor (w/an AJAX [suprise! suprise!] interface) On a side note: AJAX is really starting to annoy me with everything "AJAX Inside" BS but its just that it would work well in this situation for n00bs to do their thing.
But to start small [well still a large project] would be to create a script to do the shell script and then move on. IMO I think that the lightweight distros are the best to go with like DSL, Feather, Puppy, and Morphix are the ones I know off the top of my head.