Hi mauphie,
Our Customer Tagger app has a backdating facility (it is a paid add-on) that will allow you to run rules against historical orders in your Shopify store. You can backdate all historical orders, or just orders for a specific date range.
All you would need to do is set-up a rule within Customer Tagger to tag Customers that have purchased a specific product or variant. Once the backdate has completed, you can then filter customers in your Shopify Admin view by that tag.
We don’t have any emailing sending capability within the app, but you could export your filtered customer list from Shopify into your preferred email sending platform.
I hope that helps.
Steve