If I remember correctly, the old AIP specifically stated that nothing in the IFR rules prevented an "Unpublished" instrument approach being used outside of controlled airspace for non-public transport flights.
Around the time of the JAA changes this paragraph disappeared from the AIP.
From memory, it was a couple of years ago that the CAA proposed changing the ANO to prevent such unpublished approaches being used outside controlled airspace. After a while this idea was fortunately dropped. The ANO already requires the commander of the aircraft to ensure that the flight can be undertaken safely, and I take this to mean that you must check any unpublished instrument approaches yourself before using them.
I know of several unpublished instrument approaches which have been successfully and safely used for many years. However, I would not refer to them as DIY approaches. The criteria for instrument approaches are all published and you should not underestimate the amount of work required to design what may look like a "simple" procedure, whether it is "published" or "unpublished".
Of course, if you are talking about trying to descend to land in IMC using a hand-held GPS, then that is a different subject.....