Unverified. First, unlock the Ripper in Career Mode. Switch to to the difficulty you intend to play on, and select your desired guitarist. Switch to a different difficulty and select the Grim Ripper. Now, switch back to the difficulty you intended to play on. The poster should read "your guitarist" playing the "Scythe."

As long as you do not change your guitar or character, your guitarist will be using the Scythe on each song of that difficulty. To return your guitarist to non-lethal guitars, simply change your guitar in the "change guitar" menu.

A similar process can equip the Ripper with a mortal guitar. Move onto your preferred difficulty as the Grim Ripper. Go to any other difficulty, select your desired guitar on any character (besides the Ripper), then return to your preferred difficulty.