I've logged into our roboRio with the user 'lvuser' and no password. That's the login I found in the ant build scripts. Worked fine. I'm not sure what the permission level of lvuser is, or even if there's sudo access, or what have you, but it got me in.
As far as passing in params to the JRE to get it to make a heap dump look at the 'robotCommand' file in ~/wpilib/java/current/ant on the development machine. The ant scripts will scp (copy) that over that to the robot and by default it reads:
Code:
env LD_PRELOAD=/lib/libstdc++.so.6.0.20 /usr/local/frc/bin/netconsole-host /usr/local/frc/JRE/bin/java -jar /home/lvuser/FRCUserProgram.jar
Set your JRE params there and that command should copy over to the roboRIO with your new JAR and get you what you need.
Disclaimer: I haven't tried this... just my assumptions from poking around the build system.