Using this extras site: http://www.qa.jboss.com/binaries/RHDS/updates/development/6.0.0.Alpha2a.extras/

And selecting this plugin: Mylyn (Selected) 6.0.0.Alpha2-v20120924-2110-B28

Which shows this description:
Contains Mylyn 3.6 including support for Bugzilla, CVS, SVN, JIRA, Bamboo, Crucible, Fisheye, Java Development, Plug-In Development and Wikitext (Selected).
Features selected by Red Hat for use with JBoss Developer Studio.

Fails to install with this error:

Cannot complete the install because one or more required items could not be found.
Software being installed: Mylyn (Selected) 6.0.0.Alpha2-v20120924-2110-B28 (com.jboss.jbds.mylyn.feature.feature.group 6.0.0.Alpha2-v20120924-2110-B28)
Software currently installed: JBoss Developer Studio (Branded Product) 6.0.0.Alpha2-v20120924-2110-B28 (com.jboss.jbds.all 6.0.0.Alpha2-v20120924-2110-B28)
Missing requirement: Mylyn Task List 3.8.1.v20120725-0100 (org.eclipse.mylyn_feature.feature.group 3.8.1.v20120725-0100) requires 'org.eclipse.mylyn.discovery.feature.group [3.8.0,4.0.0)' but it could not be found
Only one of the following can be installed at once:
Mylyn Commons 3.8.1.v20120725-0100 (org.eclipse.mylyn.commons.core 3.8.1.v20120725-0100)
Mylyn Commons Core 3.6.0.v20110608-1400 (org.eclipse.mylyn.commons.core 3.6.0.v20110608-1400)
Cannot satisfy dependency:
From: JBoss Developer Studio (Branded Product) 6.0.0.Alpha2-v20120924-2110-B28 (com.jboss.jbds.all 6.0.0.Alpha2-v20120924-2110-B28)
To: com.jboss.jbds.product.feature.feature.group [6.0.0.Alpha2-v20120924-2110-B28]
Cannot satisfy dependency:
From: Mylyn (Selected) 6.0.0.Alpha2-v20120924-2110-B28 (com.jboss.jbds.mylyn.feature.feature.group 6.0.0.Alpha2-v20120924-2110-B28)
To: org.eclipse.mylyn_feature.feature.group 0.0.0
Cannot satisfy dependency:
From: JBoss Developer Studio (Core Features) 6.0.0.Alpha2-v20120924-2110-B28 (com.jboss.jbds.product.feature.feature.group 6.0.0.Alpha2-v20120924-2110-B28)
To: org.jboss.tools.central.feature.feature.group [1.1.0.Alpha2-v20120922-2327-B43]
Cannot satisfy dependency:
From: Mylyn Commons UI 3.8.1.v20120725-0100 (org.eclipse.mylyn.commons.workbench 3.8.1.v20120725-0100)
To: bundle org.eclipse.mylyn.commons.core 3.8.0
Cannot satisfy dependency:
From: Mylyn Task List 3.6.5.v20120215-0100 (org.eclipse.mylyn_feature.feature.group 3.6.5.v20120215-0100)
To: org.eclipse.mylyn.commons.core [3.6.0,3.7.0)
Cannot satisfy dependency:
From: JBoss Central 1.1.0.Alpha2-v20120922-2327-B43 (org.jboss.tools.central 1.1.0.Alpha2-v20120922-2327-B43)
To: bundle org.eclipse.mylyn.commons.workbench 3.8.0
Cannot satisfy dependency:
From: JBoss Central 1.1.0.Alpha2-v20120922-2327-B43 (org.jboss.tools.central.feature.feature.group 1.1.0.Alpha2-v20120922-2327-B43)
To: org.jboss.tools.central [1.1.0.Alpha2-v20120922-2327-B43]