There is a solution that seems to work. Move as close to the altar as you can. Then with testingcheats on, shift click on one of the plants and "set as head". Once it's on your head, then shift click it again and choose "reset (debug)". This will put your plant back on the ground. Once the event actually starts and you're in costume, you can "say uplifting things" to the plant, and it will complete your task. But it only works once the event is in swing. You can tell, because you should see the mask icon beside the "say uplifting things" interaction.