It only works as the last argument on "apply". Using quasiquote for this wouldbe a little over-the-top.
Also, almost all the other build system, even the ones using quasiquote, alsopass them at the end of the argument list. Just scons-build-system doesn't atone single place of multiple, and meson-build-system doesn't withconfigure-flags.
I'm not opposed to doing it like you suggest, but let's do it for all thebuild systems in an extra patch.
Pushed to guix master as commit f51fd97ec54a98668d63c52d8a6bd75d8dc3292e.