The way I safeguard against this is to always reverse my MH to approx 1 yard away from the Smart, then physically pull the Smart to the towball...you soon realise if it's in gear or the handbrake's on. NB the gearstick on a Smart is an electrical connection...position of the stick is no indicator of whether it's actually in gear or not (I can confirm - have had the stick in the "N" position when it was actually in gear before now).
The scenario that really terrifies me is one of the tyres giving out. I've had a puncture on the rear wheel of Smart before and it's gone down flat enough to shred the tyre. Luckily I was driving the Smart at time so smelt the burning (not so lucky that it was on a Sunday afternoon in wilds of Yorkshire moors with no mobi coverage, but that's another tale). Had this happened while I was towing, I know I wouldn't have been aware until other drivers flagged me down....can't see Smart in wing mirrors and reversing camera is focussed on towbar/A-frame/front of Smart.
There's more of us out there, incidentally. Got back from Ravenglass today and counted 6 different MHs with towcars on site.