There isn't a way to write a Collection Point plug-in to integrate something like HP BAC as the plug-in isn't documented or exposed to the developer. I'm aware of two methods to get third party data into SLM. The first is via SNMP traps or the other is to write the third party metrics to an AR vendor form and then configure the vendor form as a data source. Using traps is going to be the easiest route to go but using a vendor form is an alternative.
I have no answer for you when you have to use the CN and CP.
Alternatively let HP BAC create a CI Unavailability record when a system goes down and let HP BAC set the CI Unavailability record set to resolved when the system is available again. I know several customers using this kind of integration also with SIM/BPPM.
Then just configure an Asset Availability Service Target in SLM to meassure the system availability.
I hope this helps.