All of the finer-granularity solutions that we are aware of, where the granularity is fine enough to be used for red zones and to cover padding bytes in structures, have implementation and use costs that would restrict them to high-end machines that use (and pay for) non-standard DRAM configurations such as those needed for ECC. I could be considered if we enter the main-frame business.
Line granularity poisoning (as opposed to silent zeroing) is possible at an area, bandwidth and performance cost of a few percent. Line granularity is sufficient to detect use-after-free, but not for use as red zones.
All such schemes have a UI aspect too: the Mill is intended to runn programs straight off the net without rewrite. When there is hardware that detects use-after-free for example, we have to be wary of the reputational damage that may happen when the faulted user howls “But it works on an x86!”. We could easily be blamed for program bugs that other machines ignore. Sad, but human nature 🙂