I have the Equilibrium close contact chaps - they are to protect legs affected by vasculitis but I think they would help for mud fever too as long as the mud wasn't too liquid. Combine with a powder treatment e.g. Keratex mud fever powder?
Equilibrium chaps have worked well for me in the past, the fit is key, they have to fit like a second skin. and you need a couple of sets so you can always have one clean set to go on.