After my experiments with the iCalendar format at the beginning of 2015, including Bugzilla feeds from Fedora and reSIProcate, aggregating tasks from the Debian Maintainer Dashboard, Github issue lists and even unresolved Nagios alerts, I decided this was fertile ground for a GSoC student.
In my initial experiments, I tried using the Mozilla Lightning plugin (Iceowl-extension on Debian/Ubuntu) and GNOME Evolution's task manager. Setting up the different feeds in these clients is not hard, but they have some rough edges. For example, Mozilla Lightning doesn't tell you if a feed is not responding, this can be troublesome if the Nagios server goes down, no alerts are visible, so you assume all is fine.
To take things further, Iain Learmonth and I proposed a GSoC project for a student to experiment with the concept. Harsh Daftary from Mumbai, India was selected to work on it. Over the summer, he developed a web application to pull together issue, task and calendar feeds from different sources and render them as a single web page.
Harsh presented his work at DebConf15 in Heidelberg, Germany, the video is available here. The source code is in a Github repository. The code is currently running as a service at horizon.debian.net although it is not specific to Debian and is probably helpful for any developer who works with more than one issue tracker.