They seem to have sold a bunch of the SS's and at least some of the very few ZL1's. Wonder how many Hertz will realistically have... they don't usually do the special editions that long, but this is uncharted COVID territory.
A couple years ago I reserved a Camaro SS vert and Hertz stopped offering it at that location. My reservation showed some invalid blank stuff online, and customer service was no help. Day of, they luckily had something similar (brand new Mustang GT vert with PP1), but from what I could tell there's no guarantee and it's up to the location what they give you. Hertz special edition cars are usually "model guaranteed" reservations, so you might get lucky. If not, argue for a C63
EDIT: realized my Camaro SS vert res was model guaranteed too, so that certainly doesn't guarantee they'll actually have it