I am looking to create an Access application with occasionally connected online sync functionality. The idea is to use SQL Server Compact as a backend locally, then use a program to sync that to a server-side SQL Server. I can't find enough definitive evidence that you can't do that. Can you use SQL Server Compact as a backend for an MS Access application?
You can do that via ADO as described here.
But as you wish to replicate the data, it would be much simpler to use SQL Server Express as it can be set up right away as a replication subscriber to a database published by your server-side SQL Server.