I would create a separate Activity (i.e. nothing to do with the normal flow of the process) containing a dummy Server Event and an Email Escalation firing on the specific date (set in process level datafield) you need. Within the Escalation Action, you can expire the Activity and trough a recurring line (i.e. going back to the same activity) change the date datafield to the next date you want to send an email and re-instantiate the Activity. This will also re-instantiate the Escalation to fire on the next date specified.

Hope this makes sense,
Ockert