Yes, they are considered heavy feeders.

I think the answer depends on too many factors to say definitely yes or no. Heavy feeders compared to what?? Compared to plants that don't need you to augment the soil at all, they are heavy feeders. Compared to roses they are no more heavy feeders than the roses. It depends on the varieties you grow, how fertile the soil is naturally without you having to add anything to it, what you consider feeding the plants (do you consider adding compost, grass clippings, mulch of various sources,etc. fertilization or do you consider that as mulching). There have been years in the past when I have added nothing to my plants in terms of fertilizer for an entire growing season and they have done as well as years when I have fertilized in the spring and then again when I have cut them back and gotten them to rebloom. This spring I have not added any fertilizer to any of my potted or planted in the ground clematis and they are up and doing well. Would I consider year after year not applying fertilizer? No, but to me that tends to suggest that they are not as heavy of feeders as has been attributed of them by some.

If you answer based on what it says in the books on clematis, again you will hear contrasting ideas on how heavy of feeders that clematis are. According to John Howells in his book on the viticellas, he says that this group of plants needs little fertilization if the soil is halfway decent. Of course, what that means is entirely up to anyone's interpretation.

Overall, if I were forced into a corner and had to make a call, I would say no that they are not extremely heavy feeders if you consider the amount of fertilization they are given in my garden compared to the amount of fertilization that I give the other plants in my garden or even the lawn, shrubs, and trees.

Don't know much about aeroponic propagators so can't give you any help with that but if you successfully use it on other plants, then why not give it a go with clematis and let us know what you find out.