Trade goods and vouchers aren't the same thing as coined money, gold or otherwise.
Isn't this not a constitutional issue, but rather a legislation issue? Before the federal reserve act, plenty of states and private banks had their own currency--gold/silver certificates. Even now, exchangeable monetary instruments (bitcoin, bearer bonds, etc) are also used.
The issue becomes when the instrument in question has "US" and/or some denomination reflecting an official federal currency (dollars, cents).
A state or private entity could issue a silver/gold coin denominated in ounce Troy, or bitcoin, or whatever, and it would count as a trade good, voucher, or financial instrument, not "money". To that end, any instrument convertible to/from US dollars would also function in this regard and be legal, provided any requisite taxes due were paid (in dollars).
As with past court cases, the illegality comes into play when the instrument explicitly (or even implicitly) reflects a fixed and demandable exchange with the US federal reserve note, US minted coins, or "dollars".
Not only that, there is a big difference between coinage and reserve notes. Only the US mint can issue metallic coinage denominated in dollars. However, as a collectible or bullion item, anyone can issue a "coin" provided it doesn't say dollars, cents, or other controlled term....as evidenced by innumerable billion rounds.
The above squares with all the current case law, and your own stae,nets about trade goods and vouchers, as well as current activities.
The issue becomes when they no longer accept the US currency, or have legislation reflecting an explicit substitution.