Larry Connor, in the January 2009 American Bee Journal, page 53, writes about French beekeepers using two day old queen cells, instead of sealed cells. They modified this technique which is used by eastern European beekeepers.
They are easier/cheaper to produce. More robust to distribute. Acceptance can be quickly determined.
Before leaving my queen business, I did a little testing along this same line of thought. Using an observation hive and a few queen right and queenless nucs, I grafted larva. Put them in a starter finisher. Then removed them at various times. Put them in the ob hive and nucs. And watched how the bees reacted. And what kind of cells/queens they raised.
The youngest larva were almost always readily accepted. Acceptance dropped off when the transferred larva were older and the nucs queen right.
And the younger larva were incredibly robust. They could withstand transportation shock, initial neglect by the nuc bees, temperature variations, etc. much better than older larva or sealed cells. I think temperatures cooler than broodnest temps actually enhanced and extended the use of the youngest larva.
The only drawback, and it's common for all grafted larva, is their sensitivity to dry air. I build a carrier out of blue construction foam insulation. Drilled holes halfway through it. This allowed a wide based JZ BZ cup to set inside it while the wide base supported and sealed the opening.
I'd fill the holes with water. Give it a shake so they would be half full. And then insert the grafted cups into it.
There are some timing losses using young larva, for very short season areas. But when larger mating nucs are used, a two day old cell could get more attention than it would in a conventional queen/cell rearing setup.