Slightly different, but I saw a recipe for a fruit juice mix that was rhubarb, watermelon and lime, so may have a go at that instead. The acidity of the rhubarb should be cut by the watermelon and they are both similar in colour, the lime and peel should add some citrus notes and maybe glycerin would counteract the dry tartness. A little grape tannin because I like that sensation and may miss it.
How about this?
1kg Rhubarb, fine cut, frozen
1.5l watermelon juice.
1 lime, zest and juice
600g white sugar
1/4 tsp grape tannin
1l water to start
1tsp Glycerin
GV9 White wine yeast.
Thawing rhubarb in a bowl overnight with the sugar to draw out juices, drain, wash with water and squeeze in muslin bag....then make a crumble with apples.
Think I may add the lime zest to the frozen rhubarb at that stage for the sugar to draw out the oils, and add the juice to the mix before straining through muslin so I don't get many bits in the ferment.
Cut back a bit on the sugar as I'm aiming for 11%