Currently, replica exchange events cause all System events to be swapped between the Simulation classes. This is a dangerous design choice, as callbacks could become invalid. At the moment this isn't an issue as the callbacks are also exchanged at the same time as the System events. The problem is, this excludes all other event classes from using the handy callback.
This has now been implemented as of commit a7c3d007d128a0fdab1ffafe25e7ae7b5da3d9a2. Output plugins are still swapped between simulations and so are excluded from using these callbacks
Currently, replica exchange events cause all System events to be swapped between the Simulation classes. This is a dangerous design choice, as callbacks could become invalid. At the moment this isn't an issue as the callbacks are also exchanged at the same time as the System events. The problem is, this excludes all other event classes from using the handy callback.