The Mugen arm isn't bad, but is designed to be supported at either end. Might want to look into another PBS buggy like the Hyper series. Or the LX series has inner arm mounts more like the Maxx style. They have a turnbuckle in the middle too so you could set it up with a piece of a traxxas arm at the pillow ball end like you have now.
I don't know if you plan on running the stock front bumper or not. But what about something like this sandwhiched in the diff housing where the bumper spring goes. Excuse my poor paint skills, but I think you get the idea here for a front hingpin support. Wouldn't be too hard to make with basic tools and a chunk of aluminum.
Sorry if I'm clogging up your thread, I like brainstorming stuff like this too much!