Be sure to freeze your ice cream machine's bowl at least 24 hours in advance, if necessary.
If using fresh rhubarb, trim the ends and slice it crosswise into 1/2" pieces (see notes for frozen rhubarb).
Add rhubarb to a dry, cold, small saucepan with 1/4 tsp. salt. Turn the heat to medium and stir occasionally. When the rhubarb is mushy, stir in 2 tablespoons honey and scrape it all into a blender.
Wipe out the saucepan. Add the almond milk, coconut milk, 3 tablespoons honey and 1/4 tsp. salt to the saucepan and put it over medium heat. Stir occasionally until the honey melts evenly into the milk. Turn off the heat.
Whisk the 3 eggs yolks in a heavy medium bowl. Very carefully, begin to drizzle the warm milk mixture into the bowl as you continue to whisk the yolks. (This will temper the yolks so they come up to the same temperature as the milk; if you combined everything at once the yolks would get scrambled.) Once you've drizzled in about half the milk mixture, pour it all back into the saucepan and whisk until it's all blended.
Place the saucepan over medium/medium-low heat so it just barely simmers. Reduce the custard so it thickens, about 15-20 minutes, stirring occasionally. You might see bits of milk but that's okay.
Place a fine-mesh sieve over the blender jar (do not skip this step!) and pour the custard into the blender with the rhubarb. Discard the bits in the sieve. Add the vanilla. Blend the milk and rhubarb until smooth and pour into airtight jars/containers.
Let the rhubarb custard cool to room temperature and then refrigerate until totally cold, about 3 hours but overnight is fine.
Add the chilled custard to your ice cream machine and process per the manufacturer's instructions. You can eat it right away when it's at more of a soft-serve texture. Or freeze until, you know, frozen, about 2-3 hours. (I like to add plastic wrap directly to the surface of the gelato to help keep out air before putting the lid on.) To serve, let sit on the counter for about 10 minutes to soften.