As far as having that data show on Square's end, I believe it's tied to the same thing I mentioned earlier. I think it'll require creating the complete customer record in order to get any of that data to show in their dashboard.
I suppose you could add the custom field to the payment page and make sure the Order class stores it when saving the order, so that in your own Zen Cart admin screens you could access the cardholder name. Again, you wouldn't see it on Square's end though. You'd need to add cc_owner in the various parts of this section of the module:
https://github.com/zencart/zencart/b....php#L166-L257 akin to what's done here in this one:
https://github.com/zencart/zencart/b....php#L222-L318
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