A Platinum 1 oz trillion dollar coin will be worth a small discount to the current value of the precious metal. If Uncle Sam forced people to accept trillion dollar one ounce platinum coins as settlement of debt, the resultant inflation would render the metal content of the coin much more valuable than its face value.
My 1/10 oz gold Britannia has a face value legal tender of 10 GBP but it would be stupid to go to a shop in the UK to buy items priced at 10 GBP, as the value of its gold is currently 129 GBP. So its melt-down value is about 115 GBP..