Hello James
An alternate solution to implementing this as an array is to implement it as a linked list
Your mediator (or raher your publisher) would hold 3 refrences
the begining of the list, the end of the list and the for the current "node" and each object will have a propety holding reference to the next in line

Adding a new object is o(1) (you can verify that an object is not already in the chain by checking its refrence property - or another)
removing an object from the chain is o(n) to find it and o(1) to actually remove it
and scanning through the whole chain is also o(n)

HTH

Arnon