I think the ideal would be to make it possible to move projects among different portfolios. That way:
I can create a portfolio (X) that holds my "template" for projects
I can hide Portfolio X so that it's not visible to the public, only to admins
I can create projects in Portfolio X and then move them to a live portfolio when ready.
As it stands, looks like I'll have to use @mforny 's solution...but I'll have to create a different template in each portfolio.