There are plans to do this in the future, but this requires a lot of internal changes. The initial off heap changes are very narrow in scope and allow users to avoid GC pauses with extremely large amount of memory used (as long as entries are not excessively large themselves). To properly use mapped files/off heap to the full extent we will need to change multiple layers and also have proper support in JGroups which handles our cluster communication.
We were also hoping to take this a step further and integrate off heap/mapped files with persistent memory pmem.io: Home as well.
Unfortunately I have no idea when this would come to fruition.