dmonitor now has a webpage.
I've been running it for a night now, watching alerts come and go via manual firewall rules so I'm pretty confident it works reliably and in a way that avoids transient failures. The only obvious failure case is if each monitoring node loses the links to each of the others. (Solution there is to have a sufficiently large number of them! Hence the reason the configuration is file/directory based. rsync for the win!)
Still I will leave it there for now. The only things missing are better instructions, and more service checking plugins..
Now to go paint some more ... adding new wall decorations has inspired me a little!