runproto.py

#!/usr/bin/env python
import logging
import re
import subprocess
import sys

logging.basicConfig()
LOG = logging.getLogger()

try:
    r = re.compile(r'(?P<proto>\w+)://\[?(?P<addr>.*?)\]?:?(?P<port>\d+)?/?$')
    m = r.search(sys.argv[1]).groupdict()
    proto_map = {
        'telnet': ('telnet', m['addr'], m['port'] or '23'),
        'gopher': ('gopher', m['proto'] + '://' + m['addr'],
                   m['port'] or '70'),
    }
    subprocess.call(proto_map[m['proto']])
except Exception as e:
    LOG.exception(e)
input("Press Enter to continue...")

telnet.desktop

[Desktop Entry]
Version=1.0
Name=Telnet
GenericName=Telnet
Comment=Telnet Client
Exec=/home/terry/bin/runproto.py %U
Terminal=true
Type=Application
Categories=TerminalEmulator;Network;Telnet;Internet;BBS;
MimeType=x-scheme/telnet
X-KDE-Protocols=telnet
Keywords=Terminal;Emulator;Network;Internet;BBS;Telnet;Client;

telnet.desktop goes in ~/.local/share/applications then run:

$ xdg-mime default telnet.desktop x-scheme-handler/telnet