Summary:
It is possible to place water/lava on the block that is behind( from the player's perspective) the cobweb. Also cannot get back into bucket.
Demonstration Video
Environment
OS : Windows Vista Home Premium K SP2 32bit
CPU : AMD FX-6100 Six-Core Processor @~3.3GHz
VGA : NVIDIA GeForce 7025 / NVIDIA nForce 630a
RAM : 3070MB
Available VRAM : 1535MB
Available Storage : 141GB
Java Version :
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b18)
Java HotSpot(TM) Client VM (build 25.25-b02, mixed mode)
Confirmed. As long as you click a block not occupied by the cobweb itself (such as a block behind it), the water/lava gets placed (but can't be picked up through the cobweb).