I'm not sure about showing a number, but I have my Healbot set to change the bar color when Ra-den puts the absorb shield on everyone. Under debuffs-> custom I added "Unleashed Void" (I believe that's the one). Then I checked "bar color" and picked the color I generally use for heal absorbs. It works fine.
Create the macro as shown by Strife. You'll need one for each of the heals you're using. Then on the Spells tab in Healbot, call the macro instead of the heal directly. So, for example, if left click is Regrowth put myRegrowthMacroName instead.