We would need answers to the following questions to determine the complexity of this task, or in general to actually implement it:

• Are we only logging when a registration is enabled, or also when it's updated?
• Name of the log (first selector on Special:Log)
  • I would suggest using a generic name here, and then having subtypes (cfr. "type of deletion" input on Special:Log/delete) for enabled vs disabled; in this case, we would also need the label of this selector, and the names of its options.
• Content of the header below the page title (for the deletion log, this is "Below is a list of the most recent deletions."). Remember that this should describe all subtypes.
• Who should be able to view this log? Should be public unless there are good reasons against it.

Also, as you noted, this log would be per-wiki, and here is the technical documentation.