Specify private key in SSH as string

Try the following:

echo $KEY | ssh -i /dev/stdin username@host command

The key doesn't appear from a PS statement, but because stdin is redirected it's only useful for single commands or tunnels.

There is no such switch - as it would leak sensitive information. If there were, anyone could get your private key by doing a simple ps command.

EDIT: (because of theg added details in comment)

You really should store the key in to a temporary file. Make sure you set the permissions correctly before writing to the file, if you do not use command like mktemp to create the temporary file.

Make sure you run the broker (or agent in case of OpenSSH) process and load the key using <whatever command you use to fetch it form the database> | ssh-add -