If you have BR writer with RE media (I expect few errors from the start so better to have one rewritable media) I can send you disk image once it's ready. I will start with simple example dumping some interesting system properties, trying to create file. Later (if everything works as expected) I can add some more things (like socket handling).
From what I have read so far it seems that correct implementation should not be able to allow JVM go outside defined directory and its subdirectories on the filesystem. But yes, it should be on the filesystem and also it is possible to find the absolute path for that directory - i.e. system property is required to be setup and point to something like /BD-J/applications/ where each application stores its data.
I am reading some additional info on BD-J, MHP, OCAM, etc. - i.e. standards that govern these (only MHP has 1376 pages). I also have started to put together tools required for generating BD-J code, signing it to make it authorized, etc.