Precedent?
One could argue that, in the mainframe world, IBM's CICS is a predecessor to this approach. It allowed/required application code to make all 'system' calls to the container (CICS), not the underlying operating system. Mainframe architecture forced this approach, so that CICS could maintain control of the processor and internally dispatch the multitude of threads it was managing. This meant replicating Opsys functions such as memory management, timers, file access, and task dispatching. It was only when there was no internal work ready to run that CICS (and IDMS) would issue a 'wait' and free up the TCB/thread/processor.
CICS is not a database, it is a teleprocessing monitor of course, but Cullinet's/CA's/Broadcom's IDMS-DC is both a TP monitor AND a DBMS. IDMS-DC is architecturally similar to CICS, with the addition of the DB component to the mix. The system definitions for both the DB side and the DC portions are stored in a database (called the 'Dictionary', but no different in form than a user database). CICS has been around over 50 years (longer than Ingres), IDMS-DC more than 40 (but commercialized in bulk to the existing IDMS-DB user base while Ingres was still a pup).
Both CICS and IDMS-DC provide 'system calls' for memory management, file access, etc. Those calls are generally processed internal to the container, using a pool of resources obtained from the operating system.
One could contemplate an approach where the 'few' operating system functions and batch processes these systems need being folded into the DB itself.