SSH is your friend.
It is essentially a secure Telnet client/server. All you'd have to do is poen a port on the firewall to allow port 22 to traverse the firewall on a static mapping to the HP.
SSH can be set up to use ONLY specific (strong encrypted) keys that are pre-configured on each end (no passwords, no clear text).
The other slick thing about SSH is that most server/clients also come with an SFTP application through the encrypted tunnel, so you can transfer files (as permitted by the server configuration) either way.
You can also pass "X" traffic, or any other (specifically configured) port either way throug the encrypted tunnel.
SSH is very secure, and using PKI absolutely minimizes the security exposure.
Check out ssh.com, openssh.org, f-secure.com, and vandyke.com.
SSH is available as open source @ no cost; the commercial products have more features, support, etc.
Good Luck
Scott