In NYS does a landlord have a right to show an occupied unit to a prospective tenant if he received a notice to vacate from the current tenant and provides the current tenant with at least 24 hour notice?

Yes - when the unit is VACANT and CLEAN * take loads of pictures and use those in your marketing efforts next time. Prospective tenants now want to PREVIEW rental homes online and with photos you can attract a better class of prospects by using Craigslist, etc.