I understand enabling SSH on ESXi works, but it's not supported by VMware (and it could be disabled in the future, no one knows). VMware itself tells you to use the remote CLI tools for this.. You must have a good reason to persist the ssh solution 
Sorry for the delay, but it is not to late for any useful answer 
Here we are, there are different ways to accomplish this task. I tried all them and revert back to
the one creating SSH keys inside an ESXi box.
First of all I discovered that the minimum keysize between SSH and dropbear keys are 768 BITS! Keep this
info everytime in your mind. Then creating the root keys is simpler, than creating the SSH-keys for other non-root
users. The later task tends to be tedious but not impossible. I gonna try to explain the simpler way (root SSH-Keys), if
you need advice for creating non-root SSH keys inside an ESXi box, come back to me.
Prerequisites:
You must have enabled the SSH server to login into the ESXi box. (Look here in this community forum for detailed explanation)
You must have set for security reasons a root password anyway
Do not enable lockdown mode, otherwise you can only login via the DCUI.
If you need more security restrictions you can put at the end of the ssh row a -g allowing root only to login using the identity file, not interactive anymore!
Security advice:
Keep your keys expecially the private one secure by placing it inside encrypted filesystems or limit the usage by chmod 600 for user root
Steps:
login as root into a fresh booted ESXi box.
create an hidden directory called /.ssh with mkdir /.ssh
create the dropbear key by executing /bin/dropbearkey -t rsa -f id_rsa -s 768 > id_rsa.pub
make a copy of your private key by executing cp id_rsa Id_rsa.db
now convert the dropbearkey to SSH format by executing dropbearconvert dropbear openssh id_rsa id_rsa.ssh.
NOTE: This is your private SSH-Key the public key remains the same.In other Linux/Windows Environement copy id_rsa.ssh to id_rsa and you can use the same keys everywhere:-)Copy the public key to authorized_keys with cp id_rsa.pub authorized_keys
NOTE: Please check that ALL private keys MUST have chmod 600 otherwise every SSH server refuse to use it, because other chmod are INSECURE!If you plan to use PuTTY as I do always, copy the id_rsa.pub and id_rsa.ssh to a PuTTY environment, rename id_rsa.ssh to Id_rsa and use PuTTY to create an id_rsa.ppk (Putty Private Key) file
Copy the newly created id_rsa.ppk key to your ESXi box under /.ssh in case you need it elsewhere and forgot how to build it again
Now put all the /.ssh stuff inside the oem.tgz
Reboot and get the message file out of an ESXi box trying from another place scp -i id_rsa root@<esxi-ip>:/var/log/messages .
NOTE: If everything went fine you will never be asked to provide the root password and can now execute batch commands via cron
Try it and give me feedback, if this works for you!
BTW I created a shared document for all the other people in this community.
If this answer was helpful consider giving me some credit points! Thx.