FWIW, the old Specialized Trispokes had a design that didn't even allow for a meaningful lockring. Front and rear hubs were identical (and the same hubs as for road- just an axle swap), and to use one as a rear you screwed an adapter onto it, then the cog onto the adapter. In principle you could use an adapter that would allow for a lockring, but then you'd just spin the adapter off in the unlikely event of putting that much back pressure on. The adapter I had was kind of goofy - it didn't even have flats to use a wrench on, so I ended up using a pipe wrench on it (and carrying the pipe wrench to use as backing wrench when removing cogs).