An interesting problem. I can't think of a coupler lock which work for the Andersen Ultimate coupler, but it hardly seems worth bothering anyway, since very few people would have a ball at the height needed to tow a trailer converted to this system, unless perhaps they have a high flatdeck with a ball of the same diameter. Even with a coupler lock, I agree that a minute with a couple of wrenches (and preferably one hex key to loosen the clamping screws first) would expose the stock kingpin; an effective lock would need to cover the heads and nuts of the bolts which secure the coupler to the kingpin.
I don't think the recent (and somewhat mysterious, not published on the Andersen website) design modification to the Ultimate frame makes any difference to security. The rail-mount version attaches to the rails with four pins like any other rail-mount hitch; if those are sized the same as an available locking pin (would a 1/2" diameter locking coupler pin work?) they could be locked. I bet even a padlock to fit could be found, but padlocks are not intended for this service and may be too brittle to use as hitch pins.
If you have the "gooseneck" version (anchored to a bed-mounted ball), then perhaps some combination of holes in parts and padlocks or locking pins could make the hold-down system non-removable.
There's also the pin which holds the Andersen Ultimate ball into the supporting frame. I would check the size and see if some available locking pin (including the locks offered by Andersen
for other types of hitch) would fit. No one is likely to steal the ball for its value, but if taken by a vandal the rig would be disabled until a replacement is found. You could just remove the ball when unhitched and lock it away in the truck cab or the trailer.
It would be pretty awkward to tow the trailer away without retracting the landing gear, so a reasonable lock on the gear compartment doors would help. As with any trailer, the wheels can be secured by various methods (some effective, all inconvenient, some ineffective, and some just bad).
Of course the obvious approach would be to just ask Andersen. It seems that they no longer respond to email (I've been waiting a month for a response), and I wouldn't recommend Facebook as way of interacting with any business, so I suggest a phone call.