Hello,
If all you're doing is checking the hosts are alive I'm guessing there's some reason you can't just use ping (the default in your host definition)? That would be the trivial option. If ping is blocked by the firewall or not accessible for some other reason then SNMP would be the way to go. I have worked on projects in London and Vancouver adding network devices to Nagios, HP Openview, etc - some devices have been a lot weirder than HSM, trust me! I would love to help you with this.
I can get this done within a few hours if you accept, assuming the devices you want to connect to are reachable over the network on port 161 in both directions. I can help you with the firewall if not.
Feel free to let me know if you have any questions.
Cheers, Harry