Hello Chris,
I would personally just use the popup. However in theory you could create a validation message yourself in a div, hide the div based on a validation trigger boolean, and change that boolean in your validation microflow.
By build it yourself I mean adding a validation message into the DOM via html/javascript/jquery using the html snippet. If using javascript or jquery you'd just have to find a way to trigger it when the div is visible, otherwise you might not be able to find the div - using HTML would be a lot simpler
you can do this also with a non-popup-notification: https://appstore.home.mendix.com/link/app/2856/Appronto/Appronto-notifcation-module