Can Mail.app be configured to insert a blank line before the text of the previous message in a reply?
Not ideal but: 

Go into Preferences: Signatures, create a new Signature, erase the name and e-mail address, and hit Enter once to create a blank line. Then check the "Place signature above quoted text" box. 

Note: once a signature has been saved with actual text, Mail won't let you blank it out, so make sure you do this with a new sig, rather than editing an old one.