I see you point and I agree that what I suggest is ugly, but, as I said, for me it is better than grep, and improvement do not worth further efforts, so I'll simply continue using it by myself
Really good SBCL-specific solution would be to add some sort of lispwork's dspec facility for SBCL, so that M-. would show exactly
(swank/backend::definterface foo) (swank/backend::defimplementation foo)
I have a patch to do things like that for asdf:defsystem and SBCL's function encapsulations (advices), so I see (ASDF/PARSE-DEFSYSTEM:DEFSYSTEM :BUDDEN-TOOLS) in my M-. for :budden-tools
Code is here:
https://bitbucket.org/budden/budden-tools/src/default/sbcl--find-definition-...
https://bitbucket.org/budden/budden-tools/src/default/asdf-3.1.4-tools.lisp?...
But here's not the sbcl-devel :)