You can access the space after/before value via the scripting model. Depending on what you're after, you can change the value in the selected text or target the parent paragraph style.
theParagraph.paragraphAttributes.spaceAfter = 12
theParagraph is a variable you define based on the target you're after.
12 is the value in points.
For your purposes, you want to increment the value up or down. You'll want to first retrieve the existing value then change it according to your increment of choice. That would look something like this:
theParagraph.paragraphAttributes.spaceAfter = ( currentValue - 2 )
When you add the scripts to InDesign's scripting palette you can assign a custom shortcut.
If you're a glutton for punishment, you can read about InDesign scripting all day long in this PDF from Adobe.