Net stake weight approximation works like hashrate approximation works with PoW.
PoW diff-1 is 4.295 * 10^9 hashes per block
PoS diff-1 is 4.295 * 10^9 coin*day*second per block
But I could not find (or calculate) the number of total coin*day (or coin*day calculated from more-than-30-day-old coins only).
ABE is able to provides such stats. But it's pretty senseless, because the network doesn't make any difference between active/inactive/destroyed coins.
You can also use this chart
http://charts.novaco.in/#mat_outsto get some info.