I noticed, by watching force hosting on PS3 and Xbox, that the method they use is that they block the game from connecting to other lobbies and they modified the "Game's begin in...." timer to 0 seconds. In the game, the dvar "party_minplayers" is set by default to 2. They also freeze this value to 2 so that when someone joins the lobby, the game starts immediately. When freezing this value to 1 or 0 the lobby doesn't search for players or lobbies anymore.

If all is doable, to force host on PC we need:

1.Block the game to join other lobbies
2.Freeze party_minplayers to "2"
3.Change "Game's begin in..." timer to 1 or 0

If all that is doable, a real Force Host for PC might see the light of day.