The lack of adverse effect on the ivy etc. suggests they are not.

Not necessarily so. A number of plants are unaffected by juglone, while others (tomatoes are a well-documented example) exhibit markedly inhibited growth, even at a distance from the drip line.

I should think the same would be true of the Norway maple --and English ivy is a notoriously vigorous, hardy plant, which is one reason it is such a problematic invasive pest in many areas of the U.S.

Thanks for your input, though, bboy. CK

Weedlady, "I should think the same would be true of the Norway Maple" is stretching it. Completely not true.

Norway Maple leaves, properly installed as a mulch, will not inhibit trees and shrubs already growing there. The idea that they are allelopathic comes from the fact that little grows under a Norway Maple tree. It's the voracious roots and strong canopy that limits growth under the tree, not chemicals it produces.
I wouldn't hesitate to use those leaves as a mulch. I have, numerous times, with no ill affects.
Mike

I don't think they are. But here is an interesting study on what to do with your fall leaves. They can actually reduce Dandelions in your lawn.

Here is a link that might be useful: Killing dandilions.

The way I understand the article, the benefit of adding the chopped leaves to the lawn is primarily that of a fertilizer, making the grass grow more thickly (which I definitely have seen to be true) which then reduces the chance of any type of weed seeds germinating. In one area in the backyard of our previous home, I routinely would rake all the leaves (a mix of sugar and red maple and several oak species) into a windrow and then take the mower over them several times to chop them for the compost pile or mulch. The finest bits were cornflake-sized and were left behind to decay in the grass when I raked the larger bits into the compost. After a couple years of this, that patch of grass was noticeably thicker and greener than the surrounding area.

I do not think that the article is saying that a phytotoxin in maple leaves is preventing the dandelions from growing. A healthy lawn, properly maintained, minimizes weed growth in general. The article also pointed out that in a previous study, it had not been determined whether or not there may have been herbicide residue present that could have skewed the results.
The article states:
"The high application rate, regardless of tree genus or species, resulted in the highest green-up ratings. Common dandelion plant counts after one (2003) and two (2003 and 2004) mulch applications at the high rate showed that up to 80% and 53% reduction was achieved, respectively. Results indicate that mulching leaves regardless of genus (oak or maple) or maple species into established turfgrass as a leaf litter disposal method will increase spring green-up and contribute to a reduction in common dandelion population." [Emphasis mine.]