Have you integrated the resource monitor into HG? I like the idea of a tracking database such as rpi-monitor shows and munin talks about, but I really don't want to run a second web server to display data. Perhaps if the content were generated and somehow linked into a HG widget or run through the same web server it would be better.
I was actually just looking for a Raspi backup method a week or so back. I found a script that was supposed to create an image that was the minimum required size (no blank space stored) and a second script to expand it back out to the provided card as I recall. I'll have to dig out the link and post it. Using dd looks like a good approach but IIRC, it carries some danger of corrupted backups unless all services are stopped such that files don't change on backup. Have you seen any issues with corruption?