Hmm, it might easier for your development implementation if you only allow this rich-Text field to only be edited in the Touch UI editor. Once the user saves the dialogue changes, either an alert or an error message will be shown to the user, and not allowing the dialogue to be saved.
You can achieve this with the Granite UI Validation API.
A short and sweet example can be found here, where it examples how the multifield is validated when attempted to be saved.
https://blogs.perficient.com/2018/10/26/simple-multifield-max-item-validation-for-aem-6-4/
$(window).adaptTo("foundation-registry").register("foundation.validation.validator", {
selector: "[data-foundation-validation^='validate-url']",
validate: function(el) {
// validate will be invoked, every time when the dialogue is being saved.
// validate logic uses JQUERY to get dialogue values from the Granite UI.
// return false to dis-allow saving.
// return true to allows saving.
return isValidationCorrect($(el).text());
}
});
function isValidationCorrect(text) {
...
}
