I've been trying to develop a provably fair scheme that would work with multiple players (e.g. multiple players each betting on the same coin flip). After doing some research[0][1], I'm drawn to the conclusion that such a scheme is impossible to achieve.
Have you seen Bustabit's solution? They have many users bet on the same bet all the time. Although the site knows the outcome ahead for 10 million rolls, they can't change it anymore to adjust for players' behaviour.
See:
Bustabit (v2) Seeding Event