I added a Sitefinity backend page through the Administration -> Backend Pages tool... however, this does not show up anywhere in the Sitesync I can find, and there's no apparent way of migrating this in the Backend Pages control itself.
How do you sync new backend pages?
It is not possible to sync backend pages. You can find a detailed list of synchronized and not synchronized content here: https://docs.sitefinity.com/synchronized-content
Backend Pages is in the list "Not synchronized data", you cannot sync it.
You need to recreate it manually on your destination server