Depending on the nature of the app a suitable easter egg may be that over time the outputs become randomly but subtly incorrect. Another technique is to canary

Depending on the nature of the app a suitable easter egg may be that over time the outputs become randomly but subtly incorrect. Another technique is to canary mark outputs in some way (like a tell tale unique string in PDF/export outputs). In this way your sales team may be able to search for those outputs and put pressure on large orgs creating those files.