@alexm @Claire That looks doable with CSS:

.compose-form__publish-button-wrapper button.primary::after {
  content: "Wah!";
  display: block;
  line-height: initial;
  text-indent: 0;
}
.compose-form__publish-button-wrapper button.primary {
  line-height: 0;
  text-indent: -9999px;
}