Howdy,
I just added a new command called slime-edit-valueon `C-c E'. Here's what it does:
Prompt for a form that should be setf'able. Evaluate the form and pop up a buffer containing the value. Let you edit the value how you want. On `C-c C-c' send the edited value over to Lisp and setf it.
I've used a super-handy Emacs Lisp command like this for years and I'm surprised that I didn't think to SLIME it sooner.
Cheers, Luke