I don't think there is going to be an easy answer for this. Retaining customizations is always a bit of a challenge.
I will mention one other option. You could export the data from Splunk into something else that reproduces the report for you. I use the Google Import/Export app to send data to a Google Spreadsheet which is rendered into a report. I like this method because Splunk pushes the data out and therefore seems more secure (I'm not providing anyone or anything access to Splunk). I'm guessing this method is not ideal for you though because you would need to re-produce the customizations but I'm mentioning it just in case.