Before having read the attractive solution proposed by Scott, I considered, as proposed by Victron, to keep two lead batteries (in series to have 24v) connected in parallel to the lithium batteries with either a Cyrix Li CT or a Cyrix Li Charge associated with an Argofet diode distributor.
What annoys me:
The Mastervolt alternator (with lithium profile chosen with the Master Adjust software via the Masterbus-USB interface) has a bulk phase at 14.25 v, an absorption time at 0 and a maintenance phase at 13.25 v.
With the Li CT the coupling is done at 13.4v and the separation at 13.2v. The Victron batteries that I received (not yet installed, for the moment in my garage) have a voltage of 13.28v whereas they would be charged only at 50/60% according to Victron. Charged, the voltage will be a bit higher so I'm afraid that the Li CT will remain open when the engine is stopped (the alternator doesn't turn anymore) and that the Li batteries will discharge into the Pb batteries until they are below 13.2v and the relay opens.
For the Cyrix Li charge the connection is made at 13.7v and the disconnection at 13.4v.
So the generator will be switched off at the end of the bulk phase and will not use the 13.25v maintenance phase provided by Mastervolt, the Argofet diode generator will consume power even when stopped unless additional relays are installed.
The solution proposed by Scott with an SSR seems more interesting to me.