With appmon you can instrument basically every Java application. Some easier then others though.
It depends e.g. on the type of technology used communication. If there is an predefined Sensor Pack (like e.g. for Servlets) you are all set, if they use technologys where there are no OOTB sensors present in dynatrace you might have to define you own entry points.
That sounds promising enough to start with testing, I suppose the hardest part is to figure out where in the config to inject the agent, but I will sort that out with the App Owner.
Thanks for the info!