The sand in the bottom of the hole sounds like a good idea to improve the drainage and reduce the risk of rot.

You might need to provide more details about the sort of Iris you're wanting to hold over.

And how soon would you be able to plant in spring? (If you have a heavy soil that warms up slowly you run the risk of having your bulbs rot before they can make enough root growth to cope with the cold and damp. Many spring-flowering species actually start growing their roots in late summer. In this zone they slow down but don't stop. In colder areas they may pause for the winter then accelerate into growth as the seasons change.)

Given the numbers involved, I hesitate to suggest planting them up in containers and keeping them in a cool area to develop their roots. Two-gallon plastic buckets - even recycled ones, with at least four 1" holes for drainage in the bottom and a fairly gritty planting mix with a touch of tomato fertiliser added for the potash - but they'd take up a lot more room than baggies in the fridge...

I received a potted plant of daffodils last spring. When they faded I stored the bulbs and am wondering when they should be replanted;should they go back into a pot or should they go into the ground?