I never said not-in-memory vs in-memory was an abstraction, it is not. I is just physical (persistent on disk) vs non-physical. The (additional) abstraction could be implemented yes, and the 'many' requests by others for different persistence is kind of out of perspective, 5 vs thousands(?) of happy users?
And yes, if you want something like this now, including creating the interfaces, make it configurable, write the testcases for it, write the documentation etc (why implement interfaces if something was not meant to be pluggable) the burden is on you, or hire someone to implement it, or… Sorry for that, but it is open source and contributions are always welcome.
And it might be that some work is already done, but not open/ready yet, since some preparations are going on to run activiti 'in the cloud'