I believe the AIP used to refer to unpublished approaches. I seem to remember a paragraph at the end of a section on instrument approaches along the lines of .... nothing in the above shall prevent a non-public transport flight using an unpublished approach at an aerodrome outside of controlled airspace...
I have not used quotes, because I am quoting from memory, but I will try and find an old AIP and look it up. I notice this paragraph does not appear in the current AIP, but unless there was a corresponding change in something in the ANO (which I understand outranks the AIP) that may not be significant.
I'll try and find the reference in the next couple of days to check that my memory is not playing tricks.