After the March 10, 2010 patch that moved the spawn locations for certain rare/named mobs throughout Underfoot, I have gotten Cinderweb to spawn at approximately in-game /loc: 126, -390, 10 .
You may paste this into your arthicrex_3.txt file (or other layer):
P 390, -126, 10, 127, 0, 0, 2, Cinderweb
This is apparently one of the pre-patch spawn spots. There may be other spots where Cinderweb spawns, but I have not yet found them.
There may be a "Cinderweb" during the Stitch event in Ashengate, but if so, then there were two different Cinderweb enemies; the one in Arthicrex is not the same one.