For
anyone else, and to clarify, if you use the work-around which Dan
previously stated (using a stereo sidechain), you must use the stereo
version of the plugin ONLY within the PT insert; ie. you must use the
multichannel plug-in vs. the multi-mono plug-in.

Specifically,
if you have a mono kick and a mono bass, create a stereo aux next to
the bass track.  Buss the bass output to the newly created stereo aux
input.  Place a stereo version of the Eventide UltraChannel on the newly
created stereo aux, which is next to the bass.  From here create a
pre-fade send from the kick to an unused buss. ie. bus 128.  On the
UltraChannel, where the skeleton key icon is (upper left area of the plugin),
choose your bus. ie. bus 128.  Now engage your desired sidechain
function.  You should now be up and running.