I think to show related marks is enough. If user uses the marked commands or options, and they does not work, he should understand it by quantum ext-list command and get to know server does not support the extension. Let quantum Cli  connect
to server is a solution, but to show different help information in different kind of situation seemingly makes thing complex and sometimes confusing.

THe marks should look like below (see  [ext. xxx] after commands or options]:
gongysh@gongysh-laptop:~$ quantum -h
usage: quantum [--version] [-v] [-q] [-h] [--debug]
               [--os-auth-strategy <auth-strategy>] [--os-auth-url <auth-url>]
               [--os-tenant-name <auth-tenant-name>]
               [--os-username <auth-username>] [--os-password <auth-password>]
               [--os-region-name <auth-region-name>] [--os-token <token>]
               [--os-url <url>]

Command-line interface to the Quantum APIs

optional arguments:
  --version             show program's version number and exit
  -v, --verbose         Increase verbosity of output. Can be repeated.
  -q, --quiet           suppress output except warnings and errors
  -h, --help            show this help message and exit
  --debug               show tracebacks on errors
  --os-auth-strategy <auth-strategy>
                        Authentication strategy (Env: OS_AUTH_STRATEGY,
                        default keystone). For now, any other value will
                        disable the authentication
  --os-auth-url <auth-url>
                        Authentication URL (Env: OS_AUTH_URL)
  --os-tenant-name <auth-tenant-name>
                        Authentication tenant name (Env: OS_TENANT_NAME)
  --os-username <auth-username>
                        Authentication username (Env: OS_USERNAME)
  --os-password <auth-password>
                        Authentication password (Env: OS_PASSWORD)
  --os-region-name <auth-region-name>
                        Authentication region name (Env: OS_REGION_NAME)
  --os-token <token>    Defaults to env[OS_TOKEN]
  --os-url <url>        Defaults to env[OS_URL]

Commands for API v2.0:
  ext-list                    List all exts.
  ext-show                    Show information of a given resource
  floatingip-associate        Create a mapping between a floating ip and a fixed ip. [ext. router]
  floatingip-create           Create a floating ip for a given tenant. [ext. router]
  floatingip-delete           Delete a given floating ip. [ext. router]
  floatingip-disassociate     Remove a mapping from a floating ip to a fixed ip. [ext. router]
  floatingip-list             List floating ips that belong to a given tenant. [ext. router]
  floatingip-show             Show information of a given floating ip. [ext. router]
  help                        print detailed help for another command
  l3-agent-add-router         Add a router to a L3 agent. [ext. router]
  l3-agent-list               List L3 agents hosting a router. [ext. router]
  l3-agent-remove-router      Remove a router from a L3 agent. [ext. router]
  net-create                  Create a network for a given tenant.
  net-delete                  Delete a given network.
  net-dhcp-agent-list         List the networks on a DHCP agent.
  net-external-list           List external networks that belong to a given tenant
  net-list                    List networks that belong to a given tenant.
  net-show                    Show information of a given network.
  net-update                  Update network's information.
  port-create                 Create a port for a given tenant.
  port-delete                 Delete a given port.
  port-list                   List ports that belong to a given tenant.
  port-show                   Show information of a given port.
  port-update                 Update port's information.
  quota-delete                Delete defined quotas of a given tenant. [ext. quota]
  quota-list                  List defined quotas of all tenants. [ext. quota]
  quota-show                  Show quotas of a given tenant [ext. quota]
  quota-update                Define tenant's quotas not to use defaults. [ext. quota]
  
For options, if this option is not from the extension (if any)  which provides its command, we add mark too:
gongysh@gongysh-laptop:~$ quantum port-create -h
usage: quantum port-create [-h] [-f {html,json,shell,table,yaml}] [-c COLUMN]
                           [--variable VARIABLE] [--prefix PREFIX]
                           [--request-format {json,xml}]
                           [--tenant-id tenant-id] [--name NAME]
                           [--admin-state-down] [--mac-address MAC_ADDRESS]
                           [--device-id DEVICE_ID] [--fixed-ip FIXED_IP]  [--security-group SECURITY_GROUP]
                           network ...

Create a port for a given tenant.

positional arguments:
  network               Network id or name this port belongs to
  value_specs           new values for the port: --key1 [type=int|bool|...]
                        value [--key2 [type=int|bool|...] value ...]

optional arguments:
  -h, --help            show this help message and exit
  --request-format {json,xml}
                        the xml or json request format
  --tenant-id tenant-id
                        the owner tenant ID
  --name NAME           name of this port
  --admin-state-down    set admin state up to false
  --mac-address MAC_ADDRESS
                        mac address of this port
  --device-id DEVICE_ID
                        device id of this port
  --security-group SECURITY_GROUP
                        [ext. security-group]

....