Why not design and use a GPS approach? I believe design criteria are available from organisations like CASA in Oz.
If it is a matter of money for equipment (TSO GPS) then perhaps you could justify cost of aquisition with the amount of extra flight time required for the ILS procedure.
Also - please dont take this the wrong way as it is hard to get the full picture from your question - but would it be better to have the approach to along a track abeam the island rather than directly at it? (even with a radar image)