I would like a plugin that works with wpmu uptime. This plugin is only there to check that wordpress as a database connection. Uptime on your end will contact this database connection plugin and if it can not make a connection it would mean that wordpress has lost it's database connection and there are issues then need tending to.
At the moment uptime only monitors server response. So if there is a database issue it is not reported even though the site is down.
It's rare to get a lost database connection I know but when you are setting up a new VPS and so on these issues can occur if you have not calculated the resources correctly.
It would not need to be a complicated or bloated plugin as it only has one simple jobs to do and little tweaking your end to uptime.