GRJ, The open spaces are nice, I was actually kind of admiring that in some of the photos. You are giving me some ideas here as well! 
I ended up with 2' of shelves all the way around the perimeter (except for the ends which are open) below the layout with a wide open 2' 'isle' lengthwise, right down the middle. I actually ended up with pretty good access to everything from below for now. Maybe have to move a box or two if I want to get back in to something though. Of course the expansion still hasn't yet happened so it will probably all change then.
Another difference is I have concrete floors and a mechanics stool that lets me scoot around below the layout fairly easily. Just have to watch your head in certain places. A few cross braces still have traces of hair (my hair!) on them! 
With you having carpet the stool might be a little more difficult to maneuver down there and maybe hard on the carpet? Could still be handy to have though. They are about $20 at Harbor Freight. The rectangular one is a little lower than the round one. But the more you can do before the plywood goes down the better, that's for sure.
Also, I added a string of 5m LEDs (600 LEDs) down the center space which helps me find things down there. I put a switch in a cross brace for the top, right where I go in with my stool for convenience. That works very well. You would probably need a couple of strings or more for the size of yours, but it's really handy to have while you are down there. That would be easier to install with the top off as well. It was an after though for me, after the layout had a top and running trains. Just another thought here, but I imagine you have probably already thought of all that.
The legs and lower cross braces are also good places to mount little things like light power, term strips, etc. Below is also a good place to hide things, keeps the skillet and rolling pin away from the old noggin...
