JNLP will not allow Java Web Start to set arbitrary properties w/o knowing them to be secure.
The built in list of known secure properties dosn't include this one.
On a given client machine you can set a property in the deployment.properties file to allow this property in ther jnlp file:
(in 1.5, or 1.4.2): deployment.javaws.secure.properties=java.net.preferIPv4Stack
This will only help you out for the given client machine
One way you might work around this is to create a small Extension Installer that adds this line to the clients deployment.properties file., but you'll have to sign the extension installer.