That or just left as is, and for modders on PC to fix for at least that system.
There's already a work-around on it with http://newvegasnexus.com/downloads/file.php?id=37572. It takes a few menus, and requires NVSE, but it's better than letting Rex slowly die for me. If you're like LadyCrimson, you probably won't like this method and just try to adjust a value of companion's clothing to have poison resistance.
Can'y you just put anti-venom in their inventory? Always works for me.
That actually works? Well I guess it may if it can work for Stimpacks. It'll probably be better to have it as a "heal" method to your companion, just like the Companion wheel does with Stimpacks; less of a hassle if they do run out of Anti-Venom.