Will I be able to script an automated process to append data to feature services on AGO?
Yes. You can use the ArcGIS API for Python to append data to features services in ArcGIS Online.
Append Features | ArcGIS for Developers
How exactly you'll go about doing this depends on a couple of things - the data itself, where it's coming from, etc.
Let me know if you have any questions.
Retrieving data ...