Don't know about Red Cross, but NOLA teaches immobilization in its WFA courses. How long and how well may depend on the instructor - compared to an EMT course I think its more you walk away with an appreciation for immobilization as part of WFA, more than all the techniques for care. Since EMTs actually immobilize then transport I think glomming on to any of their refresher courses would be a good idea. The idea of effectively immobilizing and transporting someone for miles over any kind of wilderness is pretty remote, most often you might immobilize for SAR to arrive, who will re-mobilize and get them out.