Okay, let's forget about 100 players/room. Let's have 10. Is there a reliable way to make sure that the host has good enough Internet connection? Maybe switch hosts if the current one is having difficulties... I still think consistent connection quality is important.
You can aid in narrowing down problematic connections.
For instance, before entering a lobby to select a game to join, perhaps have the users select a world region first.
Like Europe, Asia, North America etc ...
Making players join games with players in their region of the world, or even country if you take it one step further.
The thing is, each peer, whether its the host or a peer, as soon as something is happening that is interfering with their connection a lot, you can get lag. Ofc when this happens to a host it could effect all peers ... but the concept is the same for that user.
He or she would be experiencing lag issues.
The best way is to design your communication mechanics smart.
Synch as few objects and variables as possible and use a messages for commands etc.
The less bandwith/communication is required, the better it could obviously perform.
A great thing about the scirra multiplayer, would a host drop out ... making another peer become host is very easy.
You could even event some checks during a round, checking latencies, and if the hosts is far worse then one of the peers, make that peer the host.