I've been running a set up like this for a couple of years on a three of my cars. On my 'best' set up, solar panels charge my OEM battery to 100% float every morning. The alternator sees the full battery and doesn't charge other than regen.
For the first ~6 miles the alternator just sits at 11.7 volts (no charge). After that (and several engine restarts) the ECU sees the battery as getting slightly low and starts to put a slight charge in at 12.4v. It then cycles lightly depending on how much power the solar panels produce. On a sunny day in the open it's possible to almost drive entirely alt. less.
In two years I've saved ~140k alternator Wh. That's around 11 gallons over ~12k miles (around 3%). However you trick the alt into not charging should give similar results (assuming your method has enough capacity to reduce your alternator duty cycle to around 20%). All the same, the difference is too small to pick up in tank-to-tank fill ups. 1.5% gains would likely be more typical if you can't charge at your destination.
At it's simplest, I just keep my Fiat battery on a garage mounted solar charger, that keeps the alternator off for the first few miles just the same, but I don't have kWh numbers for that one. Sure is a simple 'mod' though
