I tried your suggestion.

--8<---------------cut here---------------start------------->8---
(snippet
        '(begin
           (substitute* "contrib/swank-listener-hooks.lisp"
             ((":compile-toplevel :load-toplevel ") ""))
           (substitute* "contrib/swank-presentations.lisp"
             ((":compile-toplevel :load-toplevel ") ""))
           #t))
--8<---------------cut here---------------end--------------->8---

SWANK builds, but when I try to build mgl-pax, I get the following error:

--8<---------------cut here---------------start------------->8---
Unhandled SB-KERNEL:SIMPLE-PACKAGE-ERROR in thread #<SB-THREAD:THREAD "main thread" RUNNING
                                                      {10005585B3}>:
  Error finding package for symbol "*SOURCE-DIRECTORY*":
 The name "SWANK-LOADER" does not designate any package.
--8<---------------cut here---------------end--------------->8---

I guess the ASDF is incomplete.

-- 
Pierre Neidhardt
https://ambrevar.xyz/