How to use ansible-playbook --limit with an IP address, rather than a hostname?

Our inventory in INI style looks like this:

foo-host ansible_host=1.2.3.4 some_var=bla
bar-host ansible_host=5.6.7.8 some_var=blup

I can limit a playbook run to a single host by using the host alias:

$ ansible-playbook playbook.yml --limit foo-host

But I can't limit the run by mentioning the host's IP address from the ansible_host variable:

$ ansible-playbook playbook.yml --limit 1.2.3.4
ERROR! Specified hosts and/or --limit does not match any hosts

The reason I want to do that is because Ansible is triggered by an external system that only knows the IP address, but not the alias.

Is there a way to make this work? Mangling the IP address (e.g. ip_1_2_3_4) would be acceptable.

Things I've considered:

Turn it on its head and identify all hosts by IP address:
```
1.2.3.4 some_var=bla
5.6.7.8 some_var=blup
```
But now we can't use the nice host aliases anymore, and the inventory file is less readable too.
Write a custom inventory script that is run after the regular inventory, and creates a group like ip_1_2_3_4 containing only that single host, so we can use --limit ip_1_2_3_4. But there's no way to access previously loaded inventory from inventory scripts, so I don't know which groups to create.
Create the new groups dynamically using the group_by module. But because this is a task, it is run only after --limit has already decided that there are no hosts matching the pattern, and at that point Ansible just gives up and doesn't run the group_by task anymore.

Solution

Better solutions still welcome, but currently I'm doing it with a small inventory plugin, which (as opposed to an inventory script) does have access to previously added inventory:

plugins/inventory/ip_based_groups.py

import os.path
import re

from ansible.plugins.inventory import BaseInventoryPlugin
from ansible.inventory.group import Group


PATH_PLACEHOLDER = 'IP_BASED_GROUPS'
IP_RE = re.compile('^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$')


class InventoryModule(BaseInventoryPlugin):
    '''
    This inventory plugin does not create any hosts, but just adds groups based
    on IP addresses. For each host whose ansible_host looks like an IPv4
    address (e.g. 1.2.3.4), a corresponding group is created by prefixing the
    IP address with 'ip_' and replacing dots by underscores (e.g. ip_1_2_3_4).

    Use it by putting the literal string IP_BASED_GROUPS at the end of the list
    of inventory sources.
    '''

    NAME = 'ip_based_groups'

    def verify_file(self, path):
        return self._is_path_placeholder(path)

    def parse(self, inventory, loader, path, cache=True):
        if not self._is_path_placeholder(path):
            return
        for host_name, host in inventory.hosts.items():
            ansible_host = host.vars.get('ansible_host', '')
            if self._is_ip_address(ansible_host):
                group = 'ip_' + ansible_host.replace('.', '_')
                inventory.add_group(group)
                inventory.add_host(host_name, group)

    def _is_path_placeholder(self, path):
        return os.path.basename(path) == PATH_PLACEHOLDER

    def _is_ip_address(self, s):
        return bool(IP_RE.match(s))

ansible.cfg

[defaults]

# Load plugins from these directories.
inventory_plugins = plugins/inventory

# Directory that contains all inventory files, and placeholder to create
# IP-based groups.
inventory = inventory/,IP_BASED_GROUPS

[inventory]

# Enable our custom inventory plugin.
enable_plugins = ip_based_groups, host_list, script, auto, yaml, ini, toml