I just mounted a splash guard cut out of a 2L soda bottle using the two cage bolts on the underside of the downtube of my Bike Friday. Not nearly as good as a full front fender, but it lets me still fold/pack the bike in its suitcase without having to remove the guard. The concave side of the guard faces the front tire so the spray tends to be directed inward and down toward the road instead of hitting the convex downtube and then out to the side and onto my feet.

And I agree with some of the comments above about using a spare cage for holding tools and other small items. A tennis ball container fits well and holds quite a bit.