Basically you seem to (to me anyways) that you are lending them your gold to hold on to...
No. Not at all, in fact. There is no "loan" involved. It would be an allocated "holding", for a bailment fee (the actual meaning of "bank"), not a "deposit" like our current idiotic so-called "banking" system, where you implicitly agree to turn over title to your currency to the financial institution, which then "owes" you that amount (regardless if you have a "right now" claim). When you store your belongings in storage, the storage company does not take title to your property, nor can it be said that they "owe you" for these things. There are no IOU's involved, and so long as you make your storage payments, they are not free to use or otherwise encumber your property in any way.
When you need money for something YOU are the one, not "SELLING" it, but rather
converting, exchanging one form of legal tender to another. The holding institution with which you have an account only acts as an agent on your behalf for that conversion. Otherwise, it is no different than walking up to any currency conversion exchange window and converting, say, Euros for Dollars -- nothing more than a legal tender exchange. You are the one doing the conversion, regardless of the agent's intermediary role, which acts with a specific power of attorney on your behalf.
Can you withdraw your gold? You likely won't get back the exact same coin you sent in to them.
Of course you can withdraw. And getting back the same coin is not important, nor is it necessary to establish an "allocated holding". Because the coins are fungible, allocations can be shifted, but at all times the coins (and even fractions thereof) would be allocated - NOT POOLED.
Or are you restricted to businesses who will take the card and deduct a certain amount of metal- say grams of gold- and have a "grams balance" or whatever on the card and it would then be up to the merchant to figure out the exchange rate of the grams to dollars for his bookkeeping purposes? That puts the burden on the business instead of the card issuer.
If you have a PayPal Visa or Masterard, are you restricted only to sites that accept PayPal, or is VISA or Mastercard the operative word? The entire process would be completely transparent to merchants. Purchases could be made at any company that takes debit cards. Read=Most. The difference: Merchants that accept only gold and silver coin could take title to part of your holding without any FRN intermediary conversion involved. That process would also be transparent to buyers.
If a merchant accepts all forms of legal tender, you would have a choice of paying in either. If a merchant specifically prices something in gold coin, silver coin, and Fed note denominations (three "face value prices" for three different forms of legal tender), you could choose which currency (or combination) to pay in - no differently than a purchase where you pay some of it with cash, some with a gift card, and the balance on a debit card. For those merchants that only accept FRN's, you could choose to pay with the card of your choice. If PM's are taking a hit, or on a dip, pay with your normal bank debit card. If PM's are doing well, pay with that card instead.
Simple. All of it.