How to refer to $container-max-widths in Bootstrap

It's possible.

max-width: map-get($container-max-widths, "g");

You can create a function to retrieve the breakpoint values like so:

@function get-breakpoints($key: "md") {
    @return map-get($container-max-widths, $key);
}

.test-class {
    max-width: get-breakpoints("g");
}