I'd also recommend the drum scan route for this. Not sure whether LightJet prints to 5x6'--check with www.westcoastimaging.com. If not, Duggal in New York does a lot of mural printing (www.duggal.com), and I'm sure they have a few options depending on what exactly your planned usage is.
Are you sure you can't rent/borrow a LF camera for this? You can enlarge anything, but I'd want at least 4x5" if I could.
I like Astia for portraits. EPY would also be a good choice.