A simple launcher to allow you to launch minecraft with a specified displayName so a family can play on the same server with one account. Works on Linux, probably Mac OS too. Change the constants to suit your needs.

minecraft

#!/usr/bin/python

import json, sys, os, subprocess, time, signal, sys


#
# Constants
#

# First and only argument is the player's display name
playerName = sys.argv[1] if len(sys.argv) > 1 else None

if sys.platform == 'darwin':
    jsonPath = os.path.expanduser(os.path.join('~', 'Library', 'Application Support', 'Minecraft', 'launcher_profiles.json'))
    minecraftPath = "/Applications/Minecraft.app"
else:
    jsonPath = os.path.expanduser(os.path.join('~', '.minecraft', 'launcher_profiles.json'))
    minecraftPath = os.path.expanduser(os.path.join('~', 'bin', 'Minecraft.jar'))

#
# Free Functions
#

def lockFile():
    os.chmod(jsonPath, 0o444)
    
def unlockFile():
    os.chmod(jsonPath, 0o644)

def getDisplayNameParent(jsonData):
    auths = jsonData['authenticationDatabase']
    # We assume that there's only only entry in the authenticationDatabase
    # dictionary, so this should loop only once and work fine.
    for authId in auths:
        if 'profiles' in auths[authId]:
            # same here. it's a loop, but just a lazy way to get the first and
            # only dict entry
            for profileId in auths[authId]['profiles']:
                return auths[authId]['profiles'][profileId]
        else:
            return auths[authId]
        
    # If we get here, we found nothing!
    return None

def getUserName():
    with open(jsonPath) as jsonFile:
        data = json.load(jsonFile)
        
    displayNameParent = getDisplayNameParent(data)
    if displayNameParent != None:
        return displayNameParent['displayName']
    
    return None
    
def changeUserName():
    with open(jsonPath) as jsonFile:
        data = json.load(jsonFile)

    displayNameParent = getDisplayNameParent(data)
    if displayNameParent != None:
        displayNameParent['displayName'] = playerName

        with open(jsonPath, 'w') as jsonFile:
            json.dump(data, jsonFile, indent=2)

def killLauncher(launcher):
    if sys.platform == 'darwin':
        subprocess.call(['killall', 'launcher'])
    else:
        launcher.kill()

def startLauncher():
    if sys.platform == 'darwin':
        execPath = ['open', '-W', minecraftPath]
    else:
        execPath = ['java', '-jar', minecraftPath]

    # launch it!
    launcher = subprocess.Popen(execPath)

    # the callback for handling a ctrl-c
    def signal_handler(signal, frame):
        print('You pressed Ctrl+C! Killing the launcher...')
        killLauncher(launcher);
        sys.exit(0)

    # register callback for handling ctrl-c
    signal.signal(signal.SIGINT, signal_handler)

    # return the launcher
    return launcher

    
#
# Main procedure: if a player name was given, alter the file
# accordingly. Otherwise, just unlock the file so a login is
# possible. Either way, launch Minecraft.
#
if (playerName):
    unlockFile()
    changeUserName()
    # lockFile()
else:
    unlockFile()

# OK, let's try it...
# New idea: loop and sleep, checking the username every 2 seconds or so
launcher = startLauncher()
print "Watching for an auth refresh..."

while launcher.poll() == None:
    # wait for a bit...
    time.sleep(1)
    currentUserName = getUserName()

    # if the user name is None, there's no authentication at
    # all. Gotte get one before we can proceed.
    if currentUserName == None:
        print "Waiting for login..."
        continue
    
    # if the user name has switched back after an auth token refresh, let's try this again...
    if currentUserName != playerName:
        print "Hey, it looks like the display name was reset by an auth refresh, hang on, I need to restart the launcher... "
        killLauncher(launcher)
        changeUserName()
        launcher = startLauncher()