Fill a bundt pan with 5 cups pineapple juice. Place in freezer until solid, about 8 hours or up to overnight.
Place the bundt pan in a large bowl of warm water for about 10 seconds to help remove the frozen pineapple juice ring, then add it to a large punch bowl, flat-side up. Fill the punch bowl with the seltzer and the remaining pineapple juice. Scatter the cherries over and around the pineapple juice ice ring. Serve, adding rum to individual cups as desired.