Buildapp is a wonderful little tool that Xach wrote to make the process of Common Lisp standalone executables slightly easier. Although the documentation is pretty good, I had a tiny bit of trouble (1) configuring it to work with Quicklisp systems and (2) creating a multicall binary for the first time, so I figured I might as well save one or two other people on the internet a tiny bit of work:
buildapp --output ~/programming/bin/cl-multicall --asdf-tree ~/other-code/quicklisp/ --dispatched-entry /monolith:main --load-system monolith --dispatched-entry monolith/monolith:main --load-system graygoo --dispatched-entry graygoo/graygoo:mainor, more generally,
buildapp --output /path/to/multicall/binary
--asdf-tree /path/to/quicklisp/
--dispatched-entry /default-system:default-main
--load-system first-auxilary-system --dispatched-entry first-link-name/first-system-package:first-system-entrypoint
--load-system second-auxilary-system --dispatched-entry second-link-name/second-system-package:second-system-entrypoint
--load-system third-auxilary-system --dispatched-entry third-link-name/third-system-package:third-system-entrypointat which point you create links first-link-name, second-link-name, and third-link-name that all point to /path/to/multicall/binary. Executing the first-link-name link will result in the first-system-entrypoint function in the first-system-package package being invoked, and so on.
Lessons learned: first, that unless you want to use --asdf-path on literally every Quicklisp system ever, you'll need to point --asdf-tree at your Quicklisp install, and second, that you need a default --dispatched-entry argument (with no link-name before the forward slash /) in order for the build procedure to properly work.