Two concerns I'd have with sand/gravel....

First, as Rich already mentioned, this is heavy stuff...which can make rearranging enclosures quite a chore if necessary.

Second, if you decide to use a sand/gravel mix, I'd recommend either feeding the snake in a different enclosure (i.e. a bucket) or feeding f/t and placing the dead rodent on a rock inside the enclosure. There's always the rsik of impaction if the snake swallows too much sand/gravel...Yes, in the wild swallowing some substrate is a natural occurrence but too much can definitely lead to health concerns.

-Kris