Got it after some playing.
Note that you need a recent version of CC.NET, I think version 1.4.3 or newer. This is from 1.4.4 SP1.
I had first tried on 1.4.0 but the ModificationReader task doesn't exist in older versions.
<cruisecontrol>
<!--WATCH SANDBOX CONTINUOUS IS A TRIGGER TO CONTINUOUS BUILD AND INDIRECTLY FULL BUILD -->
<project name="WatchSandboxContinuous" queue="TestQ" queuePriority="4">
<triggers>
<intervalTrigger/>
</triggers>
<sourcecontrol type="your_source_control_type">
...
</sourcecontrol>
<tasks>
<modificationWriter>
<filename>mods.xml</filename>
<path></path>
<appendTimeStamp>True</appendTimeStamp>
</modificationWriter>
<nullTask />
</tasks>
</project>
<!--BUILD SANDBOX CONTINUOUS WOULD DO A FAST CONTINUOUS BUILD AND TRIGGER FULL BUILD -->
<project name="BuildSandboxContinuous" queue="TestQ" queuePriority="3">
<triggers>
<projectTrigger project="WatchSandboxContinuous" />
</triggers>
<prebuild>
<modificationReader>
<filename>mods.xml</filename>
<path>C:\Program Files\CruiseControl.NET\server\WatchSandboxContinuous\Artifacts</path>
<deleteAfterRead>True</deleteAfterRead>
</modificationReader>
</prebuild>
<tasks>
<!--Propagate modification history to next full build-->
<modificationWriter>
<filename>mods.xml</filename>
<path></path>
<appendTimeStamp>True</appendTimeStamp>
</modificationWriter>
<nullTask />
</tasks>
</project>
<!--BUILD SANDBOX FULL WOULD DO A FULL REBUILD AT NIGHT WITH ANY ADDITIONAL TASKS -->
<project name="BuildSandboxFull" queue="TestQ" queuePriority="2">
<triggers>
<multiTrigger operator="And">
<triggers>
<projectTrigger project="BuildSandboxContinuous" />
<scheduleTrigger buildCondition="ForceBuild" time="23:00" />
</triggers>
</multiTrigger>
</triggers>
<prebuild>
<modificationReader>
<filename>mods.xml</filename>
<path>C:\Program Files\CruiseControl.NET\server\BuildSandboxContinuous\Artifacts</path>
<deleteAfterRead>True</deleteAfterRead>
</modificationReader>
</prebuild>
<tasks>
<nullTask />
</tasks>
</project>
</cruisecontrol>