I want to be able to schedule an e-mail or more of them to be sent on a specific date, preferably using GAE Mail API if possible (so far I haven't found the solution). 
Would using Cron be an acceptable workaround and if so, would I even be able to create a Cron task with Python? The dates are various with no specific pattern so I can't use the same task over and over again.
Any suggestions how to solve this problem? All help appreciated
You can easily accomplish what you need with Task API. When you create a task, you can set an ETA parameter (when to execute). ETA time can be up to 30 days into the future.
If 30 days is not enough, you can store a "send_email" entity in the Datastore, and set one of the properties to the date/time when this email should be sent. Then you create a cron job that runs once a month (week). This cron job will retrieve all "send_email" entities that need to be send the next month (week), and create tasks for them, setting ETA to the exact date/time when they should be executed.
Is there a way to schedule sending an e-mail through Google App Engine Mail API (Python)?