I just stumbled across a fairly inexpensive 72V charger!
Schumacher Multi Battery Charger — 10 Amp, 72 Volt, Model# SE-1072 | Battery Chargers | Northern Tool + Equipment
This charger is manually selectable up to 72V.
Thanks to Norm's YouTube video for the lead on this one.
This charger is about $220. It's not the exact same one as in the video, but manufacturers change the look of these products all the time.