Do any lines or brokers waive single-occupancy fees on last minute bookings?

Agencies can't waive what is essentially the cost of a cabin, not a fee. The reason double occupancy pricing appears lower is you're splitting the price the cruiseline wants for a cabin in half and calling it a per person rate. Sailing alone you typically just pay both halves of that per person rate. Try Carnival sailings and look for Cat 1A cabins. On some ships these are bunkbed configured cabins and they often have a harder time getting couples to book them As a result, they have lower rates on them for single passengers. Typically 1 1/2 times the per person rate but off season it's sometimes even lower.

Cheers, Neil