I can think of a way using a rest call to get existing views, and then having an "exclusion list" that represents those that have already been alerted for. Unfortunately I couldn't find a "created" field on these dashboards so someone else might know, but I kind of like the idea of having a csv of known "alerted" dashboards. Example:
| rest /services/data/ui/views
| search isDashboard=1 NOT
[ | inputlookup knownDashboardsList.csv
| table title]
Essentially, make a rest call to obtain all dashboards, and search those that aren't in some csv that you populate when you've alerted on a new dashboard. Run this once a week or so, and then schedule a task to make sure you update the csv whenever it triggers!
Hope this helps!
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