Sometimes simpler really is better, and as NetWatchR notes, these really will need (for most applications we're talking about) some really BEEFY diodes. We could be talking tens or even hundreds of amps, and don't forget the forward voltage drop (typ. 0.7V) which means some major power losses. It's why big motor contactors will have big beefy silver contacts. Separate charge controllers, though, would be easy and cost-effective enough, for that half of the problem.
My own issue would be 'NRE' (even as an engineer, there's times that 'non-recurring' time investment gets to be too much
) I did see a good deal on a nice used contactor and grabbed it 'just in case.' But the only time doing this 'higher tech' design makes much sense is for those of us who already have some big lead-acid batteries with life left in 'em, but would really like to have something much better on hand for when TSHTF...Yeah, I'm jazzed about NiFe, too. Wouldn't do lead acid now unless it was strictly a matter of up-front $$$.