It is possible to set an "install-dir" option for a package now

This let's you completely customize the path needed for your custom
package. So you can specify paths directly on top level and are not forced
to install your packages under the vendor/ directory.

Example:

{
    "name": "my-project",
    "version": "1.0.0",
    "repositories": {
        "MyRepo": {
            "package": {
                "name": "mypackage",
                "version": "1.0",
                "install-dir": "lib/src/mypackage"
                "source": {
                    "url": "https://github.com/awesomepackages/mypackage.git",
                    "type": "git",
                    "reference": "some-tag"
                }
            }
        }
    },
    "require": {
        "mypackage" : "1.0"
    }
}

src/Composer/Installer/LibraryInstaller.php

@@ -123,6 +123,11 @@ class LibraryInstaller implements InstallerInterface
      */
     public function getInstallPath(PackageInterface $package)
     {
+        $installDir = $package->getInstallDir();
+        if (isset($installDir))
+        {
+            return $installDir;
+        }
         $targetDir = $package->getTargetDir();
         return ($this->directory ? $this->directory.'/' : '') . $package->getName() . ($targetDir ? '/'.$targetDir : '');
     }

src/Composer/Package/BasePackage.php

@@ -27,6 +27,7 @@ abstract class BasePackage implements PackageInterface
     protected $prettyName;
     protected $repository;
     protected $id;
+    protected $installDir;
 
     /**
      * All descendants' constructors should call this parent constructor
@@ -92,6 +93,16 @@ abstract class BasePackage implements PackageInterface
         return $this->id;
     }
 
+    public function setInstallDir($dir)
+    {
+        $this->installDir = $dir;
+    }
+
+    public function getInstallDir()
+    {
+        return $this->installDir;
+    }
+
     /**
      * Checks if the package matches the given constraint directly or through
      * provided or replaced packages

src/Composer/Package/Loader/ArrayLoader.php

@@ -53,6 +53,10 @@ class ArrayLoader
             $package->setTargetDir($config['target-dir']);
         }
 
+        if (isset($config['install-dir'])) {
+            $package->setInstallDir($config['install-dir']);
+        }
+
         if (isset($config['repositories'])) {
             $repositories = array();
             foreach ($config['repositories'] as $repoName => $repo) {