So, I finally made up a "ponysona", or the artist as pony. There's a bit more gray in my hair than in Slipstick's mane, but I decided to keep it simple. The object he's manipulating is a cylindrical slide rule, and his cutie mark is a regular straight slide rule.
ETA: "No, Pinkie, not SLAPstick! Put down that seltzWHARRGARBL!"