It's going to be really hard to make a generic, non-lossy mapping in a static, stateless JS script. Have you looked at Google's dataLayer as an alternative yet? The setup guide for SnowPlow + dataLayer is here in case it's interesting: https://github.com/snowplow/snowplow/wiki/Integrating-javasc...

The dataLayer approach seems really sound to us - here's hoping other tag management solutions move to something similar (or standardise on Google's dataLayer API).