Hi all,
Have you investigated this:-
http://www.jackosx.com/
It should allow you to pipe audio from Logic to VST-RTAS wrapped plug-ins in Pro Tools (Logic will talk directly to JACK; PT will not, but will do so via RTAS, VST, and the Jack-VST plug-ins).
I've not tested it myself, but if it looks workable we will make sure that VST-RTAS supports it well.