Just to close the loop here, I'll answer my own question. According to tp-link support, yes, the HS100 will return to the previous ON/OFF state after a power outage.

I bought and tested a HS100 and can now verify this is true. After power outage, the load ON/OFF state returns to that state prior to the power outage when power is restored. It does so fairly quickly, even before the HS100 has re-aquired the wifi signal.

Good news on remembering previous state, do you know how long the HS100 the "memory" is? as we have power outages for a few hours sometimes. Tnx

I'm hoping it's stored in NVRAM permanently, but I have no idea as I have not tested for more than a few minutes and have gotten no official response.