how did I test:

Step 1 - create a devstudio-discovery.xml file, file contents are:
<directory>
<!-- include JBDS Core Central discovery plugin -->
<entry url="com.jboss.jbds.central.discovery_7.0.0.Alpha1-v20130306-0101-B124.jar" permitCategories="true"/>
</directory>

Step 2 - download Central discovery plugin jar
http://www.qa.jboss.com/binaries/RHDS/updates/development/7.0.0.Alpha1c.core/plugins/com.jboss.jbds.central.discovery_7.0.0.Alpha1-v20130306-0101-B124.jar

Step 3 - Edit plugin.xml in the Central discovery plugin jar
change siteUrl="https://devstudio.jboss.com/updates/7.0-development/central/core/">
to siteUrl="http://www.qa.jboss.com/binaries/RHDS/updates/development/7.0.0.Alpha1c.core/">

Step 4 - make the devstudio-discovery.xml and jar file available via a www server (in this case in the top html dir for an httpd server)

Step 5 - Add this to jbdevstudio.ini, start JBDS
-Djboss.discovery.directory.url=http://localhost/devstudio-discovery.xml