So I am making some progress on this....

What I am currently able to do is to prevent the product from being saved by hooking into the ProductServices event:

ProductService.Saving += OnProductServiceSaving;

Then in that method, I check for an existing node using the ContentCache:

private void OnProductServiceSaving(IProductService sender, SaveEventArgs<IProduct> e)
    {
        foreach (var savedEntity in e.SavedEntities)
        {
            foreach (var content in savedEntity.DetachedContents)
            {
                var slug = content.Slug.EnsureStartsWith('/').EnsureEndsWith('/');
                var node = UmbracoContext.Current.ContentCache.GetByRoute(slug);

                if (node != null)
                {
                    var msg = new EventMessage("Category", $"The url '{content.Slug}' already exists as a Content Node called '{node.Name}'. Please change that content's node name, or this product's 'slug' property to something else.",EventMessageType.Warning);
                    e.CancelOperation(msg);

                    //TODO: add error message to back office here warning about existing url. How?
                }
            }
        }
    }

This works as expected (prevents product from being saved) but I have no idea how to add the warning alert to the back office.

I see someone else has a similar issue too

@rustyswayne any help?

is working on a reply...