Ivory 2 is an older 32bit only AU Plug... which is why it no longer shows up in the AU manager as LPX only uses the newer 64bit AU Plugins.
This also explains why Omni and RMX work because they are both available as 32 and 64bit AU plugins so they will work with LPX
Solutions?
Unfortunately 32lives is not compatible with Ivory 2 (A program to convert/wrap older 32bit plugins to 64bit versions) so the only practical solution is to use Vienna Ensemble Pro (VEP) to host Ivory 2 as a 32bit AU plugin and then host it in LPX via the VEP5 server /plugin on your Mac Pro... Not a cheap solution but then again, VEP has so many other uses apart from this one, it might be of benefit to you.... to buy it.
http://www.viennaensemblepro.com
or...
Contact Synthology and see if they have any intention of updating it to a 64bit AU version.. I could not find any reference to them saying they are or are not doing so.... but they have recently provided a 64bit AAX version for Pro Tool users so they may well be working on a 64bit AU version too now...