How do I declare a dependency that can be fulfilled by one of many differently named packages?

You can use "any": [$dependency-spec1, $dependency-spec2]. This could look like one of the following (depending on if you use a plain string dependency or a hash):

"depends" : {
    "runtime" : {
        "any" : [
            "Foo::Bar",
            "Foo::Baz"
        ]
    }
}

"depends" : {
    "runtime" : {
        "any" : [
            {
                "name" : "Foo::Bar"
            },
            {
                "name" : "Foo::Baz"
            }
        ]
    }
}

This isn't constrained to raku dependencies either. For instance to declare a dependency on either curl or wget one can do:

"depends" : {
    "runtime" : {
        "any" : [
            "curl:from<bin>",
            "wget:from<bin>"
        ]
    }
}

which would cause zef install $whatever to inform the user of the missing programs if none are found.