If anyone's feeling adventurous, here is some sort of a plugin implementation, ready for testing. Documentation hasn't been updated. Requires AGS 3.3.
https://bitbucket.org/AJA/dialogdesigner/downloads/release-1.3-plugin.zipBackup, backup, backup to be sure you don't break your valuable game. 
And here's how you use it, roughly:
1. Copy AGS.Plugin.DialogDesigner.dll to your AGS directory.
2. Run AGS, open your project.
3. Double click Dialog Designer node in the project explorer tree.
4. Select Dialog Designer.exe.
5. Write your dialog.
6. File -> Export to AGS -> Settings -> Set the dialog name. (In this version, all these settings are dialog file specific!)
7. Make sure AGS is open and your project is loaded.
8. In Dialog Designer, select File -> Export to AGS -> AGS Plugin (or Ctrl+E)
9. Switch to AGS, there should be a pop-up saying the dialog was imported successfully. You should see the dialog in your tree view, and hopefully it will work.
10. To edit a dialog that you've already imported, open the dialog in AGS and double click on Dialog Designer in the project explorer tree.
Known issues:
- If you re-export a dialog that is already open in AGS (but is not the current active editor pane), the ui is not updated. Instead you need to close and reopen the dialog pane. I didn't find a way to fix this with the current editor plugin API since it has no way of accessing all the open editor panes. The active editor pane (if it's the dialog you're re-exporting) should be correctly updated.