.ssh-config

# ~/.ssh/config
host server
        setenv SSH_CLIENT_NAME=laptop
        hostname server.example.com

env.conf

# /etc/ssh/sshd_config.d/env.conf
AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS
AcceptEnv SSH_CLIENT_NAME

mosh-who.py

#!/usr/bin/python

import subprocess
import re
from pathlib import Path
import os

def mosh_client(line, pid):
    envfile = Path(f"/proc/{pid}/environ")

    if not os.access(envfile, os.R_OK):
        print(line)
        return

    with open(envfile, "r") as envfd:
        envdata = envfd.read()
    settings = envdata.split("\0")
    ssh_client_name = "??"
    for setting in settings:
        ssh_client = re.match("^SSH_CLIENT_NAME=(.*)", setting)
        if ssh_client:
            ssh_client_name = ssh_client[1]
    print(f"{line} {ssh_client_name}")

who_output = subprocess.run(["who"], capture_output=True)
lines = who_output.stdout.decode("utf-8").split("\n")
for line in lines:
    mosh = re.match(".*mosh \[([0-9]+)\]", line)
    if mosh:
        mosh_client(line, mosh[1])
    elif len(line) > 0:
        print(line)