Current it's not possible to send tracking event offline.

So one idea would be to extend PIWIK tracking API especially for JavaScript with additional parameter about even time-stamp (optional), so inside JS, you can keep the event locally and once connecting to internet, send all the events with the event happening time.

For basic tracking, probably can do similar way.

https://forum.piwik.org/t/does-piwik-work-even-your-offline/7295/9