There is nothing special about the Assistant phone other than that it has multiple lines (one being proxy, one the intercom and the other being personal extension and possibly a 4th speed dial to Manager intercom. As long as all the phones to which the assistant logs in, have enough line buttons on it (say all phones are 7960)..the assistant should be able to move around using EM.
One problem with this i can think of is that if you are using auto answer on speakerphone for the intercom lines, when you create extension mobility profile, you may lose the auto answer function (as its a shared line).
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus