You could try it, I would mulch heavily and make sure to plant extra deep. It's a risky business at best, but if you do keep them in their pots I would still mulch heavily. I am by no means or stretch of the imagination an expert on clems. If fact, I'm pretty new to it.
But, that being said, they probably stand a better chance in the ground than still in the pots.
Have you looked at the root systems? Are they getting pot bound? If the roots look healthy, and they are leafed out well, you could plant with a couple of nodes 6 to 8 inches down underground to protect the plant against winter. It may still have a chance then. But, do it now, don't wait. Remember to keep it watered well until the ground freezes.

I do that all the time. I have pots that have been in the ground 3 or 4 years while I wait for them to get big enough to plant out, or for me to find a spot for them. I think mulching is a good idea, but I wouldn't worry about it. Of course, I don't consider myself a clem expert, either, but I do grow quite a number of them.

Each year for the last 4 or 5 years, I have heeled in a number pots of clems for the winter that did not have a large enough root system to be planted out during the regular growing season. Out of all the clems that I have handled this way, I have only lost 1 and I think the reason I lost that one was that the root system was pathetic at best when I received the clem from Chalk Hill's going out of business sale.

I would definitely make sure your plants have the proper moisture until the ground freezes - you can mulch them, but I just cover the entire pot with a couple of inches of dirt - make sure they are sunken into the ground really well. Also, make sure you have a stake or something to mark where you buried your pots so you know in the spring right where you put them.

You should be fine.