I wish to run multivariate tests on an e-commerce site that run across all product pages.

I will be setting and deciding the variations myself all I need to do is track the results in GA.

I think may be possible (although only A/B testing is available via the GA UI):

https://developers.google.com/analytics/devguides/platform/features/experiments#serving-framework

EXTERNAL – You will choose variations, handle experiment optimization, and only report the chosen variation to Google Analytics. For example, this should be used by 3rd-party optimization platforms that want to integrate with Google Analytics for reporting purposes. In this case, the Google Analytics statistical engine will not run.

However how do I configure this and push the data to GA in my page?