Paul, simple answer, As of now if you use IPC events ..you can only browse processes within your current project. You cannot browse a process in IPC which does not belong to current solution.
Solution: though it is not perfect solution, I would create a empty process in my current solution. When i need to IPC a process which does not exist in my current project, i would use the datafield in process name ( in IPC Wizard). When next is pressed, k2 will ask for design time value for IPC, use the empty process (Or any process) here.
When your process will run, populate the datafield with correct process (Project\processname). This way you can use any process as IPC even if that does not exist in your current solution.