After adding this to eclipse.ini:

-Djboss.discovery.directory.url=http://download.jboss.org/jbosstools/discovery/development/4.2.0.Alpha2/jbosstools-directory.xml
-Djboss.discovery.site.url=http://download.jboss.org/jbosstools/discovery/development/4.2.0.Alpha2/

And installing the abridged JBoss Tools - the following error is written to the .log when JBoss Central is opened:

!MESSAGE IO failure: cannot load discovery directory
!STACK 0
java.io.FileNotFoundException: https://devstudio.jboss.com/updates/8.0-development/devstudio-directory.xml
at org.eclipse.equinox.internal.p2.transport.ecf.RepositoryStatusHelper.checkFileNotFound(RepositoryStatusHelper.java:297)
at org.eclipse.equinox.internal.p2.transport.ecf.FileReader.checkException(FileReader.java:474)
at org.eclipse.equinox.internal.p2.transport.ecf.FileReader.sendRetrieveRequest(FileReader.java:431)
at org.eclipse.equinox.internal.p2.transport.ecf.FileReader.read(FileReader.java:273)
at org.eclipse.equinox.internal.p2.transport.ecf.RepositoryTransport.stream(RepositoryTransport.java:172)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.eclipse.mylyn.internal.discovery.core.util.P2TransportService.stream(P2TransportService.java:92)
at org.eclipse.mylyn.internal.discovery.core.util.WebUtil.stream(WebUtil.java:161)
at org.eclipse.mylyn.internal.discovery.core.util.WebUtil.readResource(WebUtil.java:95)
at org.jboss.tools.project.examples.internal.discovery.xpl.RemoteExternalBundleDiscoveryStrategy.loadRegistry(RemoteExternalBundleDiscoveryStrategy.java:90)
at org.jboss.tools.project.examples.internal.discovery.xpl.ExternalBundleDiscoveryStrategy.performDiscovery(ExternalBundleDiscoveryStrategy.java:117)
at org.jboss.tools.project.examples.internal.discovery.ChainedDiscoveryStrategy.performDiscovery(ChainedDiscoveryStrategy.java:62)
at org.eclipse.mylyn.internal.discovery.core.model.ConnectorDiscovery.performDiscovery(ConnectorDiscovery.java:114)
at org.jboss.tools.central.internal.discovery.wizards.ProxyWizardManager.loadWizards(ProxyWizardManager.java:101)
at org.jboss.tools.central.internal.discovery.wizards.ProxyWizardUpdateJob.run(ProxyWizardUpdateJob.java:44)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)