QJ, finally got finished readin the entire thread and I gotta say that you've done some really nice work on your 109. I just hope that when I start building on mine, whatever it is, it will at least not look like trash. I'd love to have a XLWB but there is no tellin what I actually get so I'm just gonna have to wing it.
As for your current "perdicament"....the marine grade plywood suggestion, but add a non-slip grip coating, would be your best bet. The decking is a good idea, but more expensive than I'd want to spend for what it will be, not to mention I would be really concerned about having a slick surface, whether it be mud or plain rain, 4ft above the ground. Just remember that most falls that end up in serious injury are only 4ft to 6ft, and I'd hate for you to get as far as you have on your build just to get injured and not be able to enjoy it. I would strongly suggest to use the expanded metal as that would give the best traction as well as easily and quickly shed rain and mud, however I think I can understand the reluctance to use it.
Good luck and I can't wait to see what you come up with!