Docker bridge network with swarm scope does not accept subnet and driver options

I want to control which external IP is used to send traffic from my swarm containers, this can be easily used with a bridge network and iptables rules.

This works fine for local-scoped bridge networks:

docker network create --driver=bridge --scope=local --subnet=172.123.0.0/16 -o "com.docker.network.bridge.enable_ip_masquerade"="false" -o "com.docker.network.bridge.name"="my_local_bridge" my_local_bridge

and on iptables:

sudo iptables -t nat -A POSTROUTING -s 172.123.0.0/16 ! -o my_local_bridge -j SNAT --to-source <my_external_ip>

This is the output of docker network inspect my_local_bridge:

[
    {
        "Name": "my_local_bridge",
        "Id": "...",
        "Created": "...",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.123.0.0/16"
                }
            ]
        },
        "Internal": false,
        "Attachable": true,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            ...
        },
        "Options": {
            "com.docker.network.bridge.enable_ip_masquerade": "false",
            "com.docker.network.bridge.name": "my_local_bridge"
        },
        "Labels": {}
    }
]

But if I try to attach a swarm container to this network I get this error:

network "my_local_bridge" is declared as external, but it is not in the right scope: "local" instead of "swarm"

Alright, great, let's switch the scope to swarm then, right? Wrong, oh so wrong.

Creating the network:

docker network create --driver=bridge --scope=swarm --subnet=172.123.0.0/16 -o "com.docker.network.bridge.enable_ip_masquerade"="false" -o "com.docker.network.bridge.name"="my_swarm_bridge" my_swarm_bridge

Now let's check docker network inspect my_swarm_bridge:

[
    {
        "Name": "my_swarm_bridge",
        "Id": "...",
        "Created": "...",
        "Scope": "swarm",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.21.0.0/16",
                    "Gateway": "172.21.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": true,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            ...
        },
        "Options": {},
        "Labels": {}
    }
]

I can now attach it to swarm containers just fine, but neither the options are set, nor the subnet is what I defined...

How can I set these options for "swarm"-scoped bridge networks? Or, how can I set iptables to use a defined external IP if I can't set com.docker.network.bridge.enable_ip_masquerade to false?

Do I need to make a script to check the subnet assigned and manually delete the iptables MASQUERADE rule?

thanks guys