Surely I'm not the only one. How to get a value in custom_label0 so it feeds Facebook and Google?

Shopify Partner
14 0 1

What is the easiest way to add (and remove) a value to custom_label0 such that it flows out of the main Shopify feed to facebook and google?  

We would really like to be able to do this for an entire collection at once.

We plan to use the value during catalog set creation in Facebook and Product Grouping creation in Google.

Thanks everyone!