I think it would be useful for voluntary currency to ditch the "dollar" face value for something that doesn't need to move up or down. How about a face value in "vollars"? Or something that sounds less like "dollar". An ounce of minted copper could be one "vollar", and other coins could be valued in multiples of that. Or, if an ounce of silver is used as the vollar, you could have fractional values for smaller coins and copper. Then, somewhere, there could be a published exchange rate between vollars and dollars.
I think that tying face value to something that is doomed, like the dollar, is a bad idea, and dangerous, too, as Bernard von NotHaus discovered. I'm not interested in "dollars", but in money. Value. Hmmm. So, maybe the currency could be called "Valors". (I still like my own "Silver Dubloons".)
Whatever... I even designed a symbol that could be used for a voluntary currency like the "$" is used for dollars: