Thought I would throw up a possible fix, to complete this thread....
I was having the same problem adding new targets - all seemed well except that I couldn't get my hosts to connect to them. It was frustrating me, and got added to the "something I will deal with when I have time/must do" list.
I just realized that the volumes were setup in the volume replication section as destinations, not as a source. I changed it to source, rescanned the box, and low and behold it found it connected to the target without incident.
The sad thing is that there is even a warning message that shows up about it, but at the time I was dealing with it, I had much bigger fish to fry and apparently didn't read the warning.
You are correct you cannot connect to the destination volume during replication. So if failing over to the destination volume you have to change to source (clear the metadata) and then enable the target. But make sure to switch the source to destination and remove the target.