Since Sazed currently holds Ruin, Hemalurgy is of him now. Since he also holds Preservation, calling the combination Harmony, he also has an entirely new godmetal that we saw in BoM. Duscovered by the Southern Scadrians, who call it Ettmetal and use it to power their ships, create powerful bombs as well as Allomantic grenades. This metal is explosive on contact with water, Brandon compared it with Cesium.
As we saw with the Inquisitors, Hemalurgic constructs can survive wounds inflicted by the spike and in fact, alter the receiver's biology so that they can survive being impaled. The Inquisitors' brains itself was reshaped to allow the spikes to pass through, as well as, it seems their skeletal structure and muscular structure as well as possibly a few internal organs and their circulatory system. So couldn't their body do something to accommodate a Harmonium spike?
It doesn't solve the problem of the spike degrading or getting damaged or exploding while it's still... passing through the donor, but I'm hoping you all could provide some theories for that end.
Now, assuming success of such a venture, what power(s) do(es) Harmonium steal?
Feruchemy can sidestep the godmetal's reaction with water, provided the Feruchemist takes a few precautions like covering up their metalmind so as to not expose it to water. So what would a Harmonium metalmind store?