I adapted the impale directly from the cadaver collector, rather than our more specialized version for the spear grass. Does it work for the thorn ogre? Maybe reduce the number of creatures it can pin to its frame?
Suggested damage values? It looks like the cadaver collector uses its slam damage + 1-1/2 Str for impalement (and grinding of small creatures), and half slam damage (with no Str modifier) for damage caused by its subsequent movement or successful break free, and slam damage (with no Str modifier) for failure to break free.
I'd say cut the # of impaled creatures by a quarter. Can you remind me how big the cadaver collector is? Basing damage on slam damage as suggested sounds right.
The Large elementals in the SRD all have at least 2d6 slams, but these also have greatclubs. Let's do either 1d8 slams or drop the greatclub and give them 2d6 slams.