In a small heavy saucepan, spread sugar; cook, without stirring, over medium-low heat until it begins to melt. Gently drag melted sugar to center of pan so sugar melts evenly. Cook, stirring constantly, until melted sugar turns a deep amber color, about 2 minutes. Immediately remove from heat and carefully stir in hot water. Quickly pour into 6 hot 4-ounce jars.

In a small saucepan, heat coconut milk and whole milk until bubbles form around sides of pan; remove from heat. In a large bowl, whisk condensed milk, eggs, egg yolk and salt until blended but not foamy. Slowly stir in hot milk; stir in vanilla and, if desired, rum. Strain through a fine sieve. Pour egg mixture into prepared jars. Center lids on jars; screw on bands until fingertip tight.

Place trivet insert and 1 cup water in a 6-qt. electric pressure cooker. Place jars on trivet, offset-stacking as needed. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 6 minutes.

Let pressure release naturally for 10 minutes; quick-release any remaining pressure. Cool jars 30 minutes at room temperature. Refrigerate until cold, about 1 hour. Run a knife around sides of jars; invert flans onto dessert plates.