An idea hit me, what if a Cyst immediately connected to a building can survive even if separated from the Hive? For example:
A Crag Whips mini-base defending Ventilation Harvester, Hive in Sub-Access, and some Marines sneak from Crossroads and sever the link. The Cysts between Sub-Access and Vent become unconnected and die off (no buildings other than Cysts), but the Cysts supporting the Crag, Whips, and Harvester remain alive in Vent. The Marines would have to hit the Cysts in Vent to starve off the alien structures within, which I think shouldn't be a difficult task given the range advantage.
This would also allow a Gorge to invest a Hydra to keep his Mini-Cyst alive somewhere sneaky, creating neat forward bases at the Gorge's expense.