That's a really good idea for a feature addition to NPM. Create sites, tie the sites together, create rules around site alerting. I would think it might be easy to define a "site" structure and have all alerting inside that entity follow a set of rules.
Mostly I'm thinking out loud here as to what would be useful to the customer base of Solarwinds. A lot of the really cool features and things like you describe above are relegated to the larger enterprises who can afford to roll custom solutions, or programmatically enhance existing extensible products. Solarwinds could add some great functionality and saleability to NPM by giving some of that to everyone.