Why is RAIM prediction only valid for a preflight planning?
It might be to do with the on-board almanac update (happens roughly every 5 days?) not matching the more reliable and comprehensive data available 'real world' and available in a NOTAM, that's updated much more frequently and accurately, and thus required at the panning stage legally.
Perform the approach despite the RAIM prediction (NOTAM'd) saying N/A - I'd suggest this wouldn't be allowed for a purely GNSS approach. Depends on ground-based NAVAID's and all sorts of other considerations (WAAS?) also to allow approach other than purely GNSS...maybe?.