I think you will have to get creative, especially in your zone. Burying pots inthe ground for the winter is one method, butto keep them on the patio would be a whole different challenge. Especially if the patio is elevated which would e prone to freeze easier due to the airflow beneath.

What types of plants are you trying to overwinter?

You didn't mention what kind of plants. Tender perennials will definitely have to come indoors. Cold hardy plants usually need to be kept outdoors, but in most cases you'll want to protect them in the same way snow does, or allow them to be snow covered. Some people bury the containers in the ground, but if a patio is your only option you'll want to protect the containers from direct wind and air exposure. Snow cover usually restricts how much cold a plant will have to endure. You want to emulate that.

I will be in Zone 6-7 and hope to have perennials of various types (one being irises) on a balcony, so would be unable to bury anything. Thank you for your thoughts and suggestions.

Perennials such as bulbs are much hardier in your situation then plants that keep foliage above ground like blueberries. Something to keep in mind.

I overwinter lots of plants in pots including bulbs.
I find that most important is to make sure the soil is well draining. If not, the water (from occasional thaw) will just sit on top of soil & kill/drown/rot anything in the pot.

Is balcony open to outside elements? - I mean is it recessed into the building or not? If not, it will be much warmer, creating a micro-climate. If yes, it will be cooler since it will get also more wind blowing around them, cooling the even more. You may create kind of 'corral' around your pots, lets say from 2x4, and fill with loose leaves or hay, then perhaps stretch netting over it to prevent leaves from being blown around. Still need to make sure that some snow will be getting onto the pots, both for moisture & for insulation.

It's good to know how hardy the plants are, generally if 2-3 zones hardier than your zone, they should be OK.

Last winter I lost few pots of Crocosmia since they were in just garden soil and there was water sitting on top of the it. But overwintered more than 60 pots outside (including iris planted only in one of those long plastic shallow container ment for hanging on railing...) They were all sitting directly on the ground, grouped together, without any special protection.

Just assure excellent drainage.
Rina