A lava sponge would be very useful to help prevent floods
While it's no problem to add the logic behind a lava sponge, the number of different blocks are limited by the client. One of the current blocks would have to do it then.
It's actually pretty simple to add, but a few things need to be changed:
Blocks need to be assigned an internal ID and a display ID. The Lava Sponge's internal ID would be something like -1, so it won't interfere with official blocks, but its display ID would be that of a regular sponge. Then it simply has to be assigned a LavaSpongeBehaviour.
like brettrussell said, you would need to do some command like /lavasponge
so wen you put a regular sponge on the server it would check if lavasponge=1 and it would absorve lava instead of water.
It would probably fall under the scope of an entire custom block command (/build lavasponge), which would instantaneously replace any block you build from block slot 1 with the custom block.