I'm pretty sure there are "fill this area" type plug-ins, but for fence-based gates, there exists a much more "natural" and simpler version:
The plug-in "PorteCoulissante" by Captain_Chaos allows for building redstone-controlled gates out of wooden and iron fences without any client-side modding or using the chat line commands. From the same author, there is also the "RetractableBridge" plug-in available, doing pretty much the same for (horizontally moved) bridges.
It doesn't fill an area so much as "moving" the gates in world-space, which means there needs to be enough space above or below the gate for the raised/lowered fence gatter to go to, similar how it works with real portcullises. This can limit some designs. On the other hand, it forces the gate designs to resemble real-world ones, leading to the solution looking "natural" and fitting into the world.
With SpoutCraft and the SpoutClient, both plug-ins support sound effect for moving of the gates and bridges.