You can specify -DforceContextQualifier=xxx
as an argument to the org.eclipse.equinox.launcher_xxx
run in your build.xml.
Update:
So, for example:
<buildnumber file="build.number"/>
<tstamp>
<format property="build.date" pattern="yyyyMMddHHmm" timezone="GMT"/>
</tstamp>
<property name="build.version" value="${build.date}-${build.number}"/>
<java fork="true" jar="${jar.launcher}">
<arg value="-DforceContextQualifier=${build.version}"/>
...
Sets the qualifier for a build to YYYYMMDDHHMM-num