How about two middle names? First name and middle name that you've planned and his surname as the second middle name. Then your surname as baby's last name. Example, Amanda Jane Parker Smith.
This is my suggestion, as well. It's there. It's legal. And it's also the one most likely to get dropped from the equation except on legal documents, so for practical purposes she'll be Amanda Jane Smith. If things change Parker can be brought back into play, and the lineage is acknowledged, but you're not tied to hearing it on a day to day basis.
