If I am understanding you correctly, you want people to be able to login with telnet and be redirected right to this BBS system over minicom?
In that case, you just need to make a new user account for the BBS users, and for the shell give it the location of minicom rather than Bash or whatever the default is. Then just configure the default minicom settings from the root account.
Once a person logs into that user account over telnet, minicom will immediately start and connect with the default config. Once the user ends the minicom session, it will drop them back to the login prompt.