I've not had much call in developing Python AddIns but what you were asking is something I would have approached in VB .net using an extension with an event firing on something specific.

If you look at the help file you can create a Python add-in application extension. This can hook into various events, an obvious one is on openDocument() or possibly contentsChanged(); you could get the combobox to refresh on that.

As I've never developed a Python AddIn what I am unsure about and hopefully someone can confirm is how one hooks into the combobox in your python addIn toolbar from the Application extension to update it.

I've not had much call in developing Python AddIns but what you were asking is something I would have approached in VB .net using an extension with an event firing on something specific.

If you look at the help file you can create a Python add-in application extension. This can hook into various events, an obvious one is on openDocument() or possibly contentsChanged(); you could get the combobox to refresh on that.

Below is a basic outline of me adding things to the list, in your case you would use your code to read from your text file. I hook into 3 events to improve the regular updating of the combobox.

import arcpy
import pythonaddins


class myComboBoxClass(object):
    """Implementation for PythonAddIns_addin.combobox (ComboBox)"""
    def __init__(self):
        self.items = ["item1", "item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
    def onSelChange(self, selection):
        pass
    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
        # Add text to list on click of control as suggested by Kadir
        if focused:
            self.items.append("Click me!")
    def onEnter(self):
        pass
    def refresh(self):
        pass

class myExtensionClass(object):
    """Implementation for PythonAddIns_addin.extension2 (Extension)"""

    # Note to access the combobox I use the name after the full stop in PythonAddIns_addin.combobox, so combobox
    def __init__(self):
        # For performance considerations, please remove all unused methods in this class.
        self.enabled = True
    def openDocument(self):
        combobox.items.append("FUNKY")
    def contentsChanged(self):
        combobox.items.append("WOMBAT")
    def itemAdded(self, new_item):
        combobox.items.append("ROBOT")