Ideas:

Cut the plug off a old headphone and you have a physical "mute" token that you can plug into your tablet whenever it should be silent. Although it does not seem to work with alarms.

Edit: killing alarmd does not help - it is restarted right away. https://garage.maemo.org/projects/alarmd/ might be a good starting point, although the source does not seem to be available.

It seems like there is work ongoing to open source the alarmd code: https://bugs.maemo.org/show_bug.cgi?id=3635

In the mean time, you could file a feature request in Maemo Bugzilla

Edit 2: After reading the rest of this thread, I really wonder why I haven't thought about looking for an initscript for alarmd before. Anyway.. that's the way to go (see some posts below for how to do it .