It's purely a guess, but I would think that the rhubarb and sugar would have liquid in the bowl by the morning. Like a syrup of it's own.
Isn't rhubarb sort of like celery in it's makeup? I mean... it has a lot of water contained in it. The sugar will draw that out. Alternatively, if it doesn't, you can put just sugar by itself in a pot and heat it slowly (stirring constantly) and it will melt into a syrup, right?
So, from a strictly logistical standpoint (no experience with conserves or jellies--though my mom has made a ton of jelly over the years)... I wouldn't think you would need to add any water to it. But, I WOULD expect that you would need to stand over it stirring, so that it doesn't scorch.