You've got S900 drive on 4WD, is it belt-driven? I would assume so, but I thought I'd ask!

I'm sure there's some places you can save a bit of weight even if it's just by drilling a few holes in the HDPE or what have you. Failing that, I'd make the arm cheap and easy to snap together from parts, and get a few spares made - Hardox wouldn't bend as readily, but if you don't have the weight for it, you might be able to get away with a disposable arm and some smart driving... As long as you don't let anything hit it side on, I think you'll be alright for the most part!