Have a look at this post. It contains valuable information. It also contains links to some add-ons which do something very similar to what you want to do, like my Ze Frozen Fields.
Generally, don’t replace HTML, but append it as a child:
var myDiv = document.createElement("div")
myDiv.attribute = ...
getEditorField(0).labelContainer.appendChild(myDiv)
Also in the post, read about the pattern for add-ons which work with editor_will_load_note
.
Let me know if that helps you