It is possible to create a socket from VirtualBox connected to a pty 
in Linux with socat in order to connect terminals to CO-IDRIS.

The socat and minicom packages has to be installed to run the example below.
This method is tested on Linux Mint 20.1.

Configure in VirtualBox -> Serial Ports

    Port Number: COM1
        Port Mode: Host Pipe
        uncheck Connecting to existing pipe/socket
        Path/Address: /tmp/idriscom1-socket
    Port Number: COM2
        Port Mode: Host Pipe
        uncheck Connecting to existing pipe/socket
        Path/Address: /tmp/idriscom2-socket

Start the virtual MS-DOS machine.

Connect the host socket to a serial port, and open a terminal emulator

$ socat UNIX-CONNECT:/tmp/idriscom1-socket PTY,link=/tmp/idriscom1-pty &
[1] 25581
$ minicom -b 9600 -D /tmp/idriscom1-pty
Welcome to minicom 2.7.1

OPTIONS: I18n 
Compiled on Dec 23 2019, 02:06:26.
Port /tmp/idriscom1-pty, 18:59:14

Press CTRL-A Z for help on special keys

Start CO-IDRIS in the virtual MS-DSOS machine.

The minicom screen shows now, after login:

Welcome to minicom 2.7.1

OPTIONS: I18n 
Compiled on Dec 23 2019, 02:06:26.
Port /tmp/idriscom1-pty, 18:59:14

Press CTRL-A Z for help on special keys

login: root
DOS Idris v2.37 02/Aug/85
no mail
# ls -l
-rwxr--r--  1 root          37 Jan 01 02:14 Copy
drwxr-xr-x  4 root         224 Jan 01 02:14 adm
drwxr-xr-x  2 root        1088 Jan 01 02:14 bin
drwxr-xr-x  2 root        1088 Jan 01 02:14 dev
drwxr-xr-x  4 root         272 Jan 01 02:14 etc
-rwxrwxrwx  1 root        9308 Jan 01 02:14 grep
drwxr-xr-x  3 root         384 Jan 01 02:14 lib
drwxr-xr-x  2 root         192 Jan 01 02:14 odd
drwxrwxrwx  2 root          32 Jan 01 02:14 tmp
drwxrwxrwx  7 root         112 Jan 01 02:14 usr
-rw-r--r--  1 root           0 Jan 01 02:14 x                                   
-rw-r--r--  1 root           0 Jan 01 02:14 y                                   
-rw-r--r--  1 root           0 Jan 01 02:14 z                                   
#