This is a cool idea for a project! The closest equivalent I'm aware of is Qubit OpenTag (https://github.com/QubitProducts/OpenTag).

I'm interested in adding SnowPlow support to this (https://github.com/snowplow/snowplow) - our tracking API is very similar to Google Analytics's.

We've just gone through the quite involved exercise of mapping SnowPlow to Google Tag Manager (http://snowplowanalytics.com/blog/2012/11/16/integrating-sno...) so I was a bit surprised in the code to see this mapping for GA events:

    track : function (event, properties) {
                window._gaq.push(['_trackEvent', 'All', event]);
            }

I'm a bit confused by this - how would I use analytics.js to pass through all the valid data that I can log in a GA event or indeed SnowPlow event - https://github.com/snowplow/snowplow/wiki/javascript-tracker...

I think you might be making the assumption that events consist of an event name plus arbitrary JSON envelope. This is a very MixPanelish view of the world - it doesn't really translate to Google Analytics, Piwik, SnowPlow, Omniture...