Okay, this process of experimenting and iterating has already delivered its first lesson! I forgot that the simple signmessage approach only works for taddrs, and all my ZEC was in zaddrs as of block 570,000, so now I can’t vote with it. 
Okay, I have to either wait for a future block, and I’ll unshield some of my ZEC then to vote in the straw poll, or else someone has to implement str4d’s idea for shielded signmessage.
Is anyone else gonna cast their Coin Holder’s Straw Poll vote for a codename for NU3?