FWIW, I have tentative plans to make a PLT MzScheme/MrEd backend for SLIME, and ideally get it admitted as part of the canonical SLIME distribution.
Depending on how (and whether) that goes, next on my to-do list would probably be SISC, Gambit, or Chicken. Or a portable, feature-limited REPL-based backend.
In the interim, I use Quack: http://www.neilvandyke.org/quack/
For my day job, I use SLIME with SBCL. :)