Wow, I thought the weirdest part of the rumor was the null-crit for being dead, but just as Chillyfeez said, when you start the battle dead your Crit is set to 00. This even extends to outside of battle. I tried to equip-dequip, the critical hit chance is dead. I thought maybe it was some sort of glitch that would clear by reloading the game... nope. I am beginning to wonder if this was intentional...? Let's go back and find the cause...
Alright, there is something interesting going on here. there are two crit values stored in the character in-battle status. One is the default one which changes depending on the weapon type (x2 for conventional, x3 for bow/arrow, no multiplier for bare handed) The other value is the "true crit" rate before the multiplier. This is set to 00 when you enter battle dead. Ah! Perhaps this might explain it... This value is past normal stats. When a character starts dead, these values are mostly set to 00. The issue is that the "true crit" Byte that is carried outside of battle in this "dead" multiplier section setting it to 00, and because the game uses this value to determine initial crit chance, it becomes 00. This is exactly why everyone but Cecil would lose their crit in the Zeromus fight.
This may be difficult to fix as the game never refreshes this area in battle outside of setting it to 00 for being dead... The game has already written 00 to the natural crit byte.
...Found it! That was difficult, but I found how the data is set to 00.... Alright, this is oddly specific, nothing else changes but Killing the crit rate. Nothing dealing with ATB, no nothing. This... may have been intentional, but it's such a silly design choice if so. It is a one byte change. The instructions also set speed, but that doesn't matter. The game already know the character is dead and resets speed immediately.
$03/94ED B1 80 LDA ($80),y[$7E:2003] A:0001 X:0040 Y:0003 P:envMxdIzC - Load character
$03/94EF 29 C0 AND #$C0 A:0080 X:0040 Y:0003 P:eNvMxdIzC - Are they dead/petrified?
$03/94F1 D0 7D BNE $7D [$9570] A:0080 X:0040 Y:0003 P:eNvMxdIzC - If so, skip the Speed/Crit Rate Setup
$03/9570 FA PLX A:0080 X:0040 Y:0003 P:eNvMxdIzC
Just change in Headerless ROM (1.1) - 0x194EF to 00 and that will fix this truly bizarre behavior and I bet this will also fix the AI battle's setting crit to 00.
Well, here is another glitch. IF you start the battle with a certain multiplier and switch your weapon the game will not account the new crit multiplier... but it is applied after the battle is over.
So I looked into the weapon crit rates... most weapons set it as 80, but Bows alone also set it to 80, unless it is a Bow and Arrow then it is a different value, A0. Except while dual wielding, it sets crit to x1 multiplier through a C0 exception.
EDIT: And here is an interesting oversight... when you leave battle your crit multiplier is set back to normal. But if you equip another weapon it will change in real time in the battle stats to its multiplied value. I wonder if they originally intended in showing crit chance somewhere?