You don't just do a CAT II or III ILS. Low vis procedures have to be in effect, so ATC will know. There has to be the capability for the airport to offer a CAT II or III ILS. You will have already accepted a CAT II or III ILS if the weather is below CAT I, the airport is capable and ATC are using it. ATC know what the minimums are. If your an idiot and either not qualified, or your aircraft not certified, and you accept a CAT II or III you will probably get away with it if flying for a large airline, however I expect in your PA-28 it would raise some eyebrows and you would probably get reported.