You mean run JavaLoader.exe from <exec> task? Something like this?
Code:
<exec executable="C:\Program Files\Research In Motion\BlackBerry JDE 4.1.0"
dir="path/to/cod/file" failonerror="true">
<arg value="-u"/>
<arg value="load"/>
<arg value="HelloWorld.cod"/>
</exec>
It should work, but I didn't try it myself yet.