Building city over pressure tank....good or bad idea?

Go for it, Derek. As you mentioned, you need to build it in such a way that it can be removed when the bladder tank takes that inevitable dump. We had three large tanks at work with shelving tight up against as well as over them and there was never a problem.

None of the places I lived had one of these tanks.  I was always on city water.  My builder install the tank and suspended it from the ceiling.  The original tank was about the size of a gas grill tank.  The plumber used PVC pipe which cracked and dropped the tank to the floor causing a minor flood to the unfinished part of the basement.  Luckily most of the water went into the french drain.  The tank had a small dent and I reinstalled it with some support brackets.  While I was finishing off my train room a couple years later the tank started leaking where the dent was.  Again no real damage as none of the dry wall was installed.  Now I have a smaller tank.

Just leave enough room over the valve in case you need to add a little more air to the bladder and monkey with if you need to. I wouldn't think you're asking for trouble

As long as there's enough clearance to work on the tank and -- at worst -- remove it and replace it with another one someday, you'll be fine. It's more important to have enough work space to deal with the water connections and the pressure gauge and switch down on the water line. Some sort of catchment in case of a leak isn't a bad idea, but these tanks are rarely a problem.

Looks like reasonably usable space...just don't model your Taj Mahal over it (in case she blows)!