I haven't got any experience to share, but thought I'd 'bump' this
thread for you.
So I couldn't find any links to fungi treatment preventing crown rot, mostly the advice seems to be to dry out the soil a bit by raising the plant up. Are you talking using Rhubarb as a fungicide? that would be news to me too. It sounds more like you are worried about your Rhubarb getting crown rot because of the damp ground. Hopefully someone might have an idea for you.