Putting aside the wisdom or otherwise of using non-published procedures, the legalities for a private operation seem to allow it (in the UK).
I.e. the minima for IFR in the ANO and SERA do not apply when taking off or landing without any mention of following a defined procedure.
So it would seem reasonable that a procedure established and maintained to the same standards as a published one should offer a similar level of safety. The arguments about separation from other traffic are not much different from those for operating IFR outside controlled airspace except that you are in the vicinity of an airfield and hence more likely to encounter other traffic.
So, I agree using a hastily home-brewed approach carries increased risk, but a well thought out one done to the prescribed standards should be legal and relatively safe.