I don't think it would be tha hard to compile. You'd just need the ALC or someone with access to all of the chapter databases to run reports or SQL queries.
When HQ's new GM took the reigns he had me run reports to find out how many players we have, how long they have been playing (broken into groups of years), how many have played in the past few years, how many played last season, how many have gobbles etc.
With that kind of data you can begin to formulate info on who your active player base is and even how many times they play your chapter. If you then compared that to the data from the other chapters you could get a relatively accurate set of numbers for national.
It'd take time and man power. But what doesn't?