Thanks to Rich Lowenthal's diligent research on Operabase (a website he describes as "not complete or completely accurate"), there is now an interesting approximation of performance frequency for living and some prominent recently active opera composers:
Britten: 1068 performances of 255 productions
Glass: 221 performances of 53 productions
Menotti: 124 performances of 40 productions
Heggie: 105 performances of 25 productions
Adams: 91 performances of 24 productions
Henze: 89 performances of 22 productions
Schoenberg: 74 performances of 18 productions
Barber: 39 performances of 9 productions
Davies: 39 performances of 11 productions
Knussen: 34 performances of 5 productions
Saariaho: 33 performances of 8 productions
Picker: 21 performances of 4 productions
Nyman: 17 performances of 4 productions
Muhly: 14 performances of 3 productions
Walton: 10 performances of 5 productions
Weir: 8 performances of 3 productions
Tippett: 5 performances of 2 productions
Two great ones in a special category:
Bernstein: 533 performances of 68 productions
Sondheim: 246 performances of 35 productions