When creating VSTO Add-Ins that customize the Office Ribbon, the visual Ribbon editor makes some tasks trivial. All Ribbbon controls can be accessed by id using intellisense. The RibbonEditBox.Text field can be directly queried and modified.
Everything changes when you create the RibbonXML directly.

You cannot directly query any of the runtime values on a ribbon control. You must use the OnChange callback to get the value in the box.

        public void OnChange(Office.IRibbonControl control, string text)
        {
        }

But what if you wanted to change the value of the editBox (aka change the text)? Say perform some validation, or formatting?

I tried casting the IRibbonControl object supplied by the callback to a RibbonEditBox object… This didn’t work.
I tried casting the control object as a Dynamic and accessing the “Text” property… No dice.
I looked for some means of getting the ribbon control so I could modify the Text value – nothing.

The solution:
(Which was surprisingly difficult to find)
There are three things you will need to do:
1. Within your ribbon’s class, make sure you are saving the IRibbonUI reference provided by Ribbon_Load to a private variable. I just call it “ribbon”
2. Implement the getText callback and a private string variable to hold the internal value for the editBox
3. Implement the onChange callback to perform validation, update the private variable, and use the IRibbonUI object to invalidate the editBox control. This triggers the ribbon to call the getText callback again, effectively updating the field.

An example:
This snippet when merged into the Ribbon class, will ensure that the text entered in an edit box ends with “of doom”

First, the ribbonxml part

<?xml version="1.0" encoding="UTF-8"?>
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load">
  <ribbon>
    <tabs>
      <tab id="DoomedObjects" label="DoomedObjects">
        <group id="spg" label="Stuff">
          <editBox id="txtDoomed" label="Doomed" onChange="OnChange" getText="GetText"/>
        </group>
      </tab>
    </tabs>
  </ribbon>
</customUI>

And the c# for the Ribbon class

        private string textValue;
        private Microsoft.Office.Core.IRibbonUI ribbon;


        public void Ribbon_Load(Office.IRibbonUI ribbonUI)
        {
            this.ribbon = ribbonUI;
            textValue= "";
        }

        public void OnChange(Office.IRibbonControl control, string text)
        {
            if (!text.EndsWith("of doom"))
            {
                textValue = text + " of doom";
                ribbon.InvalidateControl(control.Id);
                //Only invalidating when the value needs to change. You could invalidate after the if block as well 
            }
            else
            {
                textValue = text;
            }
        }

        public string GetText(Office.IRibbonControl control)
        {
            return textValue;
        }