Você pode fazer isso usando o AppleScript. Aqui está um rápido script (rapidamente testado) que está perto do que você quer.
#!/bin/bash
echo "tell application \"Mail\"
activate
set MyEmail to make new outgoing message with properties {visible:true, subject:\"$2\", content:\"Some Message Here\"}
tell MyEmail
make new to recipient at end of to recipients with properties {address:\"$1\"}
make new attachment with properties {file name:((\"$3\" as POSIX file) as alias)}
end tell
end tell
" | osascript
Uso: compose_email '[email protected]' 'Some Subject' /path/to/attachment.zip