I would like to package a bit of before_build
logic as a hook with my plugin. Is it possible and if so, how?
Yes. From http://cordova.apache.org/docs/en/edge/guide_appdev_hooks_index.md.html
As a plugin developer you can define hook scripts using elements in a plugin.xml like that:
<hook type="before_plugin_install" src="scripts/beforeInstall.js" />
<hook type="after_build" src="scripts/afterBuild.js" />
<platform name="wp8">
<hook type="before_plugin_install" src="scripts/wp8BeforeInstall.js" />
<hook type="before_build" src="scripts/wp8BeforeBuild.js" />
...
</platform>