Archive Extractor Use this download and the text file in the description to extract Pack0.dzip in witcher 2/CookedPC.
Find the Endrega models. They'll be .w2ent files and the endrega are called endriag in the game files. Find different .w2ent files and rename them to the endriag filenames. For example you could use triss_naked_wet.w2ent. Be sure to change the endriag queen and arachas files too. I'm not sure if the textures are packaged in these files so they may look very odd but not like spiders anymore.
Finally move the files to your game directory under the CookedPC folder using the same folder structure that the extractor did and they should over-ride the in game models.
I haven't tested this so tell me how it goes.
EDIT: Actually I did exactly this and found numerous unresponsive nude, and headless, Triss's in the woods around Flotsam. I guess the change causes the monsters to bug out. If there was another monster model that moved on all fours that might work but I couldn't find one. To my knowledge no one can extract the .w2ent file yet so I don't know what else to try.
This didn't cause my game to crash so you could do this with a .w2ent file of some small object and never complete The Engrega Contract. There is an Arachas boss though so you'd have to grin and bear it for that.