From Misha (Misha Belilovskiy) of Autodesk who posted this trick in a reply to a discussion thread in the public Autodesk discussion group for AutoCAD 2005.
You can't do it using AutoCAD UI but it is possible if you are not afraid editing your Tool Palette source ATC file. Here is how you do it.
If you would like to replace Auxiliary scale in tool properties (for instance with LTSCALE) you can do the following:
1. In AutoCAD Tool Palette right click on the block you want to change, choose Properties and assign DIMSCALE Auxiliary scale.
2. Close AutoCAD and use Windows Explorer to navigate to the source Tool Palettes ATC files.
3. Open ATC file with the name of your palette in Notepad or your favorite ASCII editor.
4. Find DIMSCALE assigned to your block and replace it with LTSCALE.
5. Save the file, open AutoCAD and check your Auxiliary scale value. It should now be LTSCALE. You are done.