Actually, I have the same problem with "amassed" as I did with "heaped up"--namely, it still seems to be a second active verb with "the nation" as its subject, rather than a second modifier for "its darkness." Whatever the rules about the hyphenation of before-the-noun and behind-the-noun modifiers, a hyphen would still underscore the fact that "heaped up" IS a modifier. There are times to break the rules for clarity's sake, and this may be one of them--especially since there's another noun in between.
Also, "heaped-up" seems closer to the spirit of a word that Rilke apparently coined for the purpose; it's more unusual and attention-getting than the duller "amassed," especially in the context of a non-physical thing like darkness. (There's a nice bit of alliteration between "hold" and "heaped," too.)
Don't mind my argumentativeness--whatever you want to do is fine. I'm just thinking with my mouth open.
By the way, my girls and I absolutely loved this charming and informative book about Venice (with special attention to the four horses) when we were homeschooling:
We gave away most of our homeschooling-related books to other families as soon as we finished with them, but I really wish I had hung onto this one, because when I wanted to reread it before my short-notice trip, no copies were available.