I have been pondering Sands Machine BTC couplers for some time--
I like the way they facilitate breaking a bike down for travel. That
said I really want to know how you deal with the theft issue--once they
are installed you can only lock the rear triangle to something--any one with
a pipe wrench could probably force open the couplers and take the front
half of your bike, even if all they really get is a wheel and the parts (not
the complete frame).

I searched the Sands Machine website with google site search--
there is only one mention of theft, referring to someone stealing
a bike that has been broken down.

Is there an obvious answer to this? Can you somehow lock the couplings
once they are on? What do you do? I mean it is great to travel with
your bike but not if you lock it to a pole and someone steals half of it.

Thieves aren't the brightest folks in the world. My guess is they'd have no idea what an S&S coupler was, nor how to disconnect it.

If you're really worried, I'd have the builder who installs your couplings provide some mechanism for securing the front half of the bike. Perhaps a small plate gusset between the head tube and the top or down tube. Drill a hole in it, attach a padlock through the hole and then run a cable lock back to the lock that secures the back half of the frame.

Thanks--that sounds like a good idea.

This is your solution: https://www.fortebikes.com/gallery5_details.html

See the middle picture titled the "stainless 'A' hole"