This is the kind of stuff that keeps me up at night...
We are not seeing a problem with a single coupler on a monotube as it has been done successfully by other builders and with an upright our options to provide extra bracing are there.
Oddly enough... if you look at my 20 the doubled brace behind and below the joint does not provide support and some versions of this bicycle do not have it. That folding joint is strong enough to support riders who weigh several hundred pounds and they are rock solid.
We will also look at non S&S couplers for this reason.