Please refer to the source for the example plugins, and use the API to get the results you desire.

For a similar example, check out piwik/misc/api_community_sticker.php

See the code in https://github.com/piwik/piwik/blob/master/misc/api_community_sticker.php

This is a simple php script that calls the API to request 2 simple data points (visitors for the last 2 days, and number of downloads for a specific file), and prints them out.

Could this also be done by developing a custom plugin and then integrate it using EmbeddingWidgets ?

People wanting this feature would then only have to include the javascript tag on their pages.

Maybe this could be a new kind of widget: widgets which are exclusively intended to be included on tracked websites to provide visiting users information about the site they're browsing ?

Compared to existing widgets, those widgets would be contextualized according to the page they are integrated to or according other parameters.