Naively, I'd say to write it as short text and sign it with my public key, but that was under the assumption that my recent memories were being wiped.

If all my memories are being wiped there isn't really a "me" to send the message to in any reasonable sense. Even if it's just all my episodic memories I would see amnesiac!ilzolende as a member of my in-group but not as me.

I suppose I could try to package the message with a blood sample (with a lock on a timer), any sample coercively obtained would have much higher cortisol levels (edit: or show signs of sedatives or something).