You might find this a worthhile read about such an approach to filesystems at the filesystem level https://www.usenix.org/legacy/events/sec2001/full_papers/bau...

Though shred and other tools to delete via random overright do exist, but as you imply a low level approach would be much more secure.

Another consideration is the type of storage, then there are backups which will still have deleted data. Let alone SSD's which are a whole different breed and can transparantly make a block as dead and realocate some of the reserved data storage without you knowing and with that leave the existing data permently inplace for forensics. With that having an excrypted file system would certainly help cover such issues, though encryption within encryption could be the extra layer of plausable denability you require.