You don't need a clearance to land, if there is no ATC in the tower. What you do need, apparently (though I have no reference) is a clearance to fly the approach.
For a G-reg, it is not illegal to fly a DIY (unpublished) IAP. And yes this is another reason why GPS approaches are moot in many cases, especially if they don't deliver lower minima than the existing NDB IAP (Shoreham is one example).
For an N-reg, unpublished IAPs are banned by FAR 91.175 and this appears to apply worldwide.