While I like the suggestion, the issue with managers choosing the all stars is, some leagues have little to no real GMs and are bot teams. While my league will end with about 6 to 8 real GMs, at the all-star break we had 3 or 4, and those guys sign on once a week or less and don't even set their lineups, much less look around the league to pick all stars.
It may work at the higher levels, though. And I do like the idea.