Great question — answering as the UphillTech team:
I actually created Shopify Node API and was just now happened upon this months later but thought I'd answer for anyone else coming along. Take a look at the shopify API here: https://docs.shopify.com/api/metafield#update You can update the metafield directly by performing a PUT request against the metafield resource instead of the variant like so: /admin/metafields/#{id}.json You would of course need to know the ID of the metafield first so that would require a call to the variant first or you could simply store the id in your local database for reference.
Full answer by ChrisC: https://stackoverflow.stackexchange.com/a/29245178 (CC BY-SA 3.0).
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