Lithium batteries like to be charged from a constant current supply and they want it to shut down at the proper rest voltage as the current decreases. For my 13S battery, that would be 54.6 volts.
You might get your DC-DC converter to output the right voltage, but can you have it shut down the current to prevent overcharge? I think Unterhausen's advice for the regular lithium charger on an inverter is smart. A loss in efficiency isn't a big deal here. Perhaps it would be in a situation where every electron had to be hoarded.
The charger will probably run on DC if it's high enough. A 12-72 DC-DC converter might suffice instead of an inverter.