A diode would waste too much power and get very hot. You'd want to wire it so that the relay turns off when the PCS turns off, disconnecting your buffer battery from the PCS.
Schottky diodes exist. Using a MOSFET as a diode might be even better. Both of these options reduce the power loss (heat issue) from "very" to "some" and may not even need a heat sink, let alone a fan. Considering how electricity flows, there's no way I can tell to turn off the relay when the PCS turns off. When the relay is closed, the output and input are shorted together... so it becomes a feedback loop and the relay would stay closed from the buffer battery if the PCS turns off.
The pre-charging with the buffer battery might be more challenging, since you have to pre-charge the buffer battery to match the voltage of the PCS, otherwise it will trip the PCS by drawing too much current or it will charge the buffer battery with too much current.
Doesn't tend to happen. Yeah, there's a bit of a surge sometimes as the PCS just dumps near-unlimited current into the poor battery... and that could be a concern for sure. But in my experience the worst I've seen was when the PCS was at 14.4v and the battery was at 12.5 (50% charge or so). It saps the PCS for a while... but has never tripped it. Using Scan My Tesla, I see it briefly gulps 1400w, then within seconds 1100w, 800w... and back to normal. Scan My Tesla is essential for this kind of hackery, so you can see how the car is reacting to things.
Also, is the PCS voltage too high to float charge a lead acid battery? I thought I saw it was 14V.
No, the PCS voltage is constantly adjusted in order to maintain/test/manipulate the front 12v battery. The PCS and the 12v lead battery are connected by an E-fuse (which is also software controllable), not by another level of charge control. So the car tweaks the PCS voltage to keep the 12v battery charging at 10 amps, then once the battery reaches 14.4v, it backs off to idle level. So in effect, the attached lead 12v buffer battery is just "along for the ride" with the main 12v battery, at voltage levels designed to keep a lead battery very happy. Just with hundreds of amps available at that voltage.
This is rather complicated. Maybe we should just work on designing a chademo inverter and get 400V from the charge port.
Done, lol.
Vehicle To Home V2H 6kW | SETEC POWER - I used this with my Leaf long ago, and it's a massive piece of crap, but it does what it says on the tin (at least for a little while before its crappy built-in solar inverter throws a permanent fault that makes it a expensive brick). The problem isn't that "nobody's done it", it's that Tesla specifically refuses to allow power export through the DC port in any way. I guess because they have Powerwalls to sell and every one of their customers has infinite money or owns their home.
(you know, it's a damn shame we can't use emoji here!)