Don't know about Puppy, but in Fatdog you have real multi-user so you can create/use other userids.
Creating a userid and /etc/passwd normally points to what shell that will use by default, the last entry for that userid line in the /etc/passwd file. You can however create your own shell for that, a simple script, maybe just
#!/bin/sh
while :;do
readln
done
saved as /usr/bin/my-shell or whatever and set /etc/passwd for the userid to that (/usr/bin/myshell), has to be executable permissions
chmod +x /usr/bin/myshell
Some systems don't permit just any such shell being used unless its also listed in /etc/shells
If you've setup sshd (ssh server), created maybe userid ssh whose shell is that myshell script then when they login that script is run and in that case just repeatedly reads a line ... and that's all.
Extend it as you like, maybe to test what was typed in and do some action in response.
echo -n "# "
read -n 1
case $n in
g) echo goodbye;;
h) echo "you're looking for help";;
esac
... type code within the while loop perhaps. read -n 1 reads in just a single character
Read up about terminal escape codes where you can set the font colour, text position, or cat somefile.ans where that file contains content/ansii ... etc and you're on the road to creating a very basic BBS or control script.
Set your router to allow ssh traffic through, and you can access that from anywhere. Usually under the port forwarding option where you set port 22 (ssh) to be directed to a particular IP within the local net such as 192.168.1.5
To access from anywhere however you need to know your external IP address, not the 192... number but the likes of whatever https://whatismyipaddress.com/ indicates as being your IP.
As ever however be mindful that others may try to hack into your local network if/when you open up ports.
ssh is encypted data flow, in older times people used just telnet that wasn't encrypted. If the content being transferred isn't critical then it might not matter if others can see it. Some versions of busybox include telnet and telnetd (server) as a possible and simpler alternative to using ssh.
See if you can start
telnetd
and connect to it
telnet localhost
If so then you might like to create a userid for that and set its shell script ... as above.
telnet mytelnetuserid@localhost
...where mytelnetuserid userid has your script set as its shell instead of the usual /bin/sh
...
