Just about any menubar command for most OS X applications can be given a keyboard shortcut, or the shortcut changed, in System Preferences > Keyboard > Shortcuts > App Shortcuts . Your labels have names, and the names are “commands” in DEVONthink’s Data > Label menu. So, open System Preferences and give a shortcut to the name of any of your labels.
For example, I have a label named “Attention”. In the right side of the image, below, you can see where I used System Preferences to assign a shortcut to the “Attention” command – be sure to spell things exactly correct. And I show you what the result looks like in DEVONthink in the center of the image.
You can assign shortcuts to menu items in the OS X preferences. However, since you have Keyboard Maestro, that is also a simple way to do what you want. Make sure that DEVONthink is running, launch the Keyboard Maestro app, and add this shortcut, selecting the label that you want. Repeat for each of the labels.
Optionally, you can assign each of the 7 macros to the same hot key trigger and Keyboard Maestro will give you an menu to select from. If you have named your macros ‘1-Label name1’, 2-Label name2’, etc., you can assign the label by the number prefix.