You can descend to any height for the purpose of a landing, on an unpublished instrument approach
Provided that the instrument approach is in accordance with the appropriate requirements i.e. ICAO Doc 8168. It would then have an appropriate DA(H) or MDA(H) so you could not "descend to any height".
Of course the approach ban would apply if there was not the required minima for the approach and thus unless there was met reporting at the destination, the (expensive) approach procedure could not be started.
I wonder why there are so few "private" instrument approach procedures.........could be that the cost of the navigation aid, the design of the procedure, the flight checking and the met reporting simply make it not worthwhile for the few times it would be used in anger.
With no instrument approach meeting the ICAO requirements then -
IFR rule = 1000ft above everything within 5nm of the aircraft.
Not applying
when visual below 3000ft amsl or when taking off or landing.
If visual, you can see where you are going and you don't need ADF or GPS.
If taking-off or landing I hope you are visual unless you have a private CAT3 ILS system.
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The one consistent argument from IO540 in support of GPS is that IO540 finds (in this case) ADF tracking and setting the DI too difficult, it is outdated and something for those morons who can actually do it.
Even the UK uses both Transverse Mercator and Lamberts Conforman Conic for the CAA VFR charts so one needs the know those two projections hust for UK VFR operations........perhaps we will leave the others and sad things like grid navigation to us morons who actually go places.
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To answer the original question;
S-I-D = Select, Identify and Display
For crossfix -select navaid closest to along track as possible first and plot.
Regards,
DFC