OpenCmis will be really helpful for many ecm systems but it is not a one common answer for all of them.
For example, for IBM CM or Filenet there is IBM CMIS which comes bundled with Content Navigator. Please note this is also lacking some basic functionalities and i developed that just recently. (on the latest version)
I would recommend to build the java application using the most relevant CMIS apis you need and for the closest specific ECM product.If you want to build it in a way which works on all ECM platforms then the coding needs to be in different way.
like - identify which ecm system is being called by your java program
and then load the classes or functions for that specific system.
Hope the above helps :)