By max altitude notation I mean having a very large, immediately visible number, on each bit of airspace; a number that summarises all levels of tiered airspace and gives me an altitude I can fly to without entering anything that is restricted.

I originally saw this as a additional file for XCSoar (heights layer), and have adapted it for use in the flight app I currently use. It means that I can just look at the moving map and can see immediately how high I can fly without entering any airspace. Using this I don't have to wait to be warned - which seems to me is analogous to driving a car along a route and not having speed limit signs, but just a warning when you are getting too fast !

Also I don't have to double check any control zones under TMAs etc (eg multi-levels of airspace). As far as I'm concerned the less I have to look down inside the aircraft while I am flying the better. I don't use SkyDemon at the moment in flight, but I find SkyDemonLight invaluable for planning. If this idea could be implemented (as a chart option) I would probably buy the subscription. 

The Virtual Radar at the bottom of the screen makes it pretty easy to see at what level airspace begins above you.

Being picky, airspace begins a fractional of a millimetre above the surface. CONTROLLED airspace is shown very clearly on the virtual radar, as is restricted airspace which generally starts at ground level anyway. It is easily possible to get a clearance to fly in controlled airspace, and even in some restricted airspace, so there really isn't a single "maximum altitude" that could be displayed.