Excellent job Heidi!!
I think HBM as you point out is a good place to start.
For the intervention, including testimonials and stories from women who have used the Doula services can be powerful… especially if the Doula-endorsing women are representative culturally/economically/ethnically to the women you are targeting.
For social-ecological framing… think about what type of educational messaging you might provide women in community forums and at home, vs. in the clinic prior to their OB visit vs. at L&D.