Start simmering water in a saucepan. The saucepan needs to be big enough to fit a bowl on top but not so big that the bowl will touch the boiling water.
Meanwhile, whisk eggs, sugar, lilikoi pulp, and lemon zest in a heat-proof bowl and place over simmering water. (CAUTION: the bowl will get hot, handle with oven mitts.) Keep heat at medium and whisk constantly. This will ensure that everything cooks evenly and it won't get lumpy. Keep whisking until the mixture thickens and reaches about 170 degrees F. This will take about 10-15 minutes. You can test doneness by coating the back of a wooden spoon and running your finger through it. It if leaves a clear path, it is done. If not, keep cooking and whisking! (Note that it will continue to thicken as it chills.)
Carefully remove the bowl from the heat and the strain the mixture through a sieve over a medium sized bowl. Add cubed butter in 2-3 batches, whisking to completely melt and incorporate the butter in-between additions. Whisk in the tablespoon of lilikoi seeds (if desired). There is no such thing as overwhisking- it makes it light and airy! Allow curd to cool to room temperature.
Place a piece of plastic wrap over the bowl, pressing the wrap directly on the curd, and place in the refrigerator until ready to use. The plastic wrap will prevent a film from forming on the curd. (See notes for making ahead.)