I'm not sure I agree with much of this. A properly designed system won't overcharge a battery. Voltage is voltage. If there is load from any source the alternator only sees the resulting voltage and responds accordingly. Seat heat, defrost, fans, lights, etc. all vary and the electrical system load, some of them cyclically, and the system isn't grumpy at all. Allowing the 180A factory alternator to do it's thing is sufficient. The BCDC will prioritize starting battery charge and only take the excess. Similarly, it will monitor the battery it's charging and only deliver what is required for the selected profile. I've installed both a BCDC and Manager 30 and they work seamlessly. You could have a 1000A alternator and it would continue to do it's thing whether charging a 55Ah single or a 300Ah array.
A 100Ah battery is more than sufficient for a Land Cruiser that moves every 3 days. And this will charge in 2-3 hours from empty which is under the typical overlander drive day. I suppose an exception would be using electricity to cook.
If your discussion is regarding using solar as an reduncancy strategy in lieu of a second alternator, then, yes, you would need to have some battery in the system to excite the alternator field. As long as the solar could get the battery above whatever the minimum voltage is for the engine circuits to run then it would act as a surrogate charging source. If the starting battery was truly roached, one could jumper from the house battery to get things going. As
@Sandroad suggest, I doubt a 100W panel (6A) would be sufficient. My guess is 200-300W would be required. Though, that doesn't mean a 100W panel would be useless. You'd just need to perform a drive-charge-drive shuffle to allow charging to catch up with consumption.