PJB,
There is no reason you have to use plywood/Homasote to cover the whole layout. You can use it under the track only and use foam everywhere else. Or use foam inserts wherever you want something you can remove easily. Foam makes a great pop-out platform for buildings and other landscaping elements, even mountains. All you have to do is add supports for it beneath the layout. You cut out a piece of the plywood/Homasote between the grid rails, add some new support rails or crossmembers so the pop-out is even with the rest of the layout and you're good. It'd be similar to how ceiling tiles are hung and can be easily removed.
