Here is what I think : [if I understand your ? correctly]
What is the probability of the subsystem being replaced with a better/enhanced/moreCompatible system in future ?
If its very high or even moderate I will go with a design that will provide me minimal coupling. E.g., you may add an abstract layer that will be implemented by the future submodule design to interface with the system you are designing. For using the current submodule you will have to write adaptor(s).
As indicated previously JCA is an abstract layer that provides you (& containers) a universal access mechanism to data sources. JCA may be done for RDBMS and thus replace JDBC. Yes that is an abstract layer already available for use. But writing the implementation for JCA for accessing your sub module will not be a trivial task. And ask yourself is it really required to be so general
I will prefer to create my own simple to implement/use layer whenever possible.