ClassLoader problems when used as OSGI bundles

When I try to do a new JcrConfiguration(), a NoClassDefFoundException is raised (it can't find the class org.modeshape.graph.mimetype.ExtensionBasedMimeTypeDetector).

Seems to me that the Class.forName uses a ClassLoader that can't look into the installed bundles.

The content of the current thread classloader is the following:

interface org.apache.sling.launchpad.base.shared.Notifiable
class org.apache.sling.launchpad.app.Main
class org.apache.sling.launchpad.app.Main$2
class org.apache.sling.launchpad.base.shared.Loader
class org.apache.sling.launchpad.app.Main$1
class org.apache.sling.launchpad.app.ControlListener
class org.apache.sling.commons.osgi.bundleversion.BundleVersionInfo
class org.apache.sling.commons.osgi.bundleversion.FileBundleVersionInfo
class org.apache.sling.launchpad.base.shared.LauncherClassLoader
class org.apache.sling.launchpad.base.shared.Loader$1
class org.osgi.framework.Version
interface org.apache.sling.launchpad.base.shared.Launcher
interface org.apache.sling.launchpad.api.LaunchpadContentProvider

I patched the code into the class ExecutionContext so that I could override the method getClassLoaderFactory and pass a new clf to the class org.modeshape.graph.mimetype.MimeTypeDetectors, but now I have another exception when trying to create a new instance of the found class:
Could not initialize repository org.modeshape.common.SystemFailureException: java.lang.IllegalArgumentException: The stream argument may not be null
at org.modeshape.common.component.ComponentLibrary.newInstance(ComponentLibrary.java:318)

Thanks for any help you can give me.
Ale