Similar policy is used in Linux for disk space: 10% of disk space is reserved for root.
PS. Linux OOMKiller already do something similar: it just kills a process to free some memory.
No OOM recovery is fool proof. What if the OOM situation is caused by another process constantly consuming memory? As soon as you free your 1MB overhead, the other process could immediately consume it and again you're in an OOM state.
Granted, the likelyhood of this is slim, but then again, so is an OOM situation period. In the world of virtual memory and swap space, it pretty much takes a rogue process to trigger an OOM error in the first place.