How can I specify a "Text Field" as an "Attribute Field Setting" for a Product Variation in Drupal Commerce?
6/5 Follow up: 

Looking into this further, the "Cart" module calls the "Options" module in order to determine which fields are eligible to be "Add to Cart" form attributes: 

// If the current field instance is not locked, is attached to a product
// type, and of a field type that defines an options list... 


In the "Options" module, it looks for data that is entered/stored as an array, e.g. a select list, taxonomy list, etc. 

Thinking this through, it seems like it won't be possible to call the user-entered data in a text field for multiple Product Variations because it isn't stored in req'd format. 

There may be a way to write custom code to work around this, but I relented and made an absurd list of "Product Specific Options" that takes care of the use case. Since most products on the site will be imported, the burden on editor/user won't be too high.