What we do in a
Critical Mass situation when this arises is a manoevre known as
corking - the first riders through (when the light's still green) dismounting, leaving the flow of the ride for a while and planting their bodies and bikes, in calm posture, forming a barricade a few feet in front of the champing rows of cars which would otherwise enter an intersection in use by Critical Mass.
Yes, this is illegal. Still, as a Hazardous Obstacle (okay, car-framed commuter) it's in your advantage to have to wait for the whole mass only once through one or two lights rather than to split us up and have to wait for eight or nine minimasses of a few hundred people each as they roam the city erratically, trying to reconnect with each other.
Some tips from the CM glossary on proper execution of this manoevre:
* Best if accompanied by smiles and eye contact, or signs that say "Thanks for waiting!" and "Honk if you love bikes!"
* Corking a thinned section of the ride undermines its own legitimacy by tempting the aggression of the car drivers being corked. (They no longer see a thicket of bikes but are still blocked!)