This can only be done in Cadence 6.1.x and is not be hard to do. Personally, I'd rather use bindkeys either to set common layers, or to toggle the visibility of the layer palette. However, to create a toolbar of common layers could be done like this:
1. Create a directory in your home directory:
mkdir -p ~/.cadence/dfII/toolbars/byApplication
2. Create a file called Layout.toolbars with content like this:
callback "leSetEntryLayer(list(\"metal1\" \"drawing\"))"
You can have as many items as you want.
3. Create a directory for your layer icons:
mkdir -p ~/.cadence/icons/24x24
4. Look at some sample icons from Cadence ($CDSHOME/share/cdssetup/icons/24x24/). You can choose one of these, or edit it in your favorite picture/icon editor. You could even grab a screenshot of the layer from your layer palette and use that. Create an icon file called metal1.png (portable network graphics image file) in your new 24x24 directory.
5. Restart Cadence, open your layout editor and enable your new toolbar by right clicking in the toolbar area and choosing the new "MyLayers" toolbar.