The shop we've been in the last years has several solid benches scattered around the room, and each year when our robot gets to the point where we want access to both the top and bottom, we put the robot across two of them, with a gap in the middle. Then we just slide in underneath on a rolling platform to get at everything underneath. Of course we have to be careful to keep the robot from sliding off into the gap, but this is easy enough (traction wheels basically do that for you). And then we have an open benchtop on either side to do work on, and can easily access all levels of the robot - underneath, middle, and up top (standing on the bench).
Here's a photo that shows basically what I'm talking about
:
https://picasaweb.google.com/1029613...4435839820 50
Safety is always top priority, so make sure that whatever you do, your heavy robot is not going anywhere and people working on the robot aren't in any danger.