The Macy quote is a good find, and indeed there are some Google hits for 'rumouring' and 'rumoring' in a verbal sense. Virtually no generalizations about language are absolute, and exceptions can always be found. However, it's probably still true that most of us don't use 'rumo(u)ring' as a verb. You have to wade through pages of Google hits to find genuine uses. (And CGEL documents Standard English, not all dialectal or idiolectal variations.)
There's another common use, the noun, as in rumouring on the Internet. This doesn't count.
Cosntructions with expletive object 'it' and a prepositional phrase with 'abroad', 'about' are normally conscious or subconscious echoes of Shakespeare. However, if someone said they would rumour it about that something happened, without knowing the Shakespeare quote, then that too would be a genuine instance of a living form of the verb.
I should mention this is listed in the CGEL (along with several others) with this peculiarity of having only one verb form. What I just discovered was a convincing reason it couldn't be an adjective (if you allow these constructions without 'to be', and I agree they don't sound perfectly natural).
Great post qroqqa, I hadn't thought about it, but this is indeed a very interesting word. Both the cases you list which you consider counterexamples seem to me to have an implicit "to be" in them, "rumored to be completely furnished" and "rumored to be dead". In fact, it doesn't sound wholly grammatical to me if I hear rumored outside of "to be".
A highly unusual verb in Present-day English: it has only this one verb form. Although it was historically a full verb with all its parts ('Come hither Catesby, rumor it abroad, That Anne my Wife is very grieuous sicke.'—Richard III, IV.ii), for most of us today it can only be a past participle.
This raises the question of why it should be counted a verb at all, rather than an adjective: compare 'she was rumoured to be dead', 'she was keen/eager/reluctant to be dead': adjectives can take infinitival clause complements.
Well last night I found the answer, when I read this sentence opening Dorothy Parker's 'Mrs. Hoftstadter on Josephine Street':
That summer, the Colonel and I leased a bungalow named 947 West Catalpa Boulevard, rumored completely furnished: three forks, but twenty-four nutpicks.
'Completely furnished' is an adjective phrase (AdjP), and adjectives can't take AdjP complements, but verbs can: compare *'eager/easy/pleasant completely furnished' with 'considered completely furnished'. And indeed, on checking Google this morning, I find quite a few "was rumoured dead"—not the way I'd say it myself (I'd much prefer to add 'to be'), but common enough to prove it's verbal in Standard English. So, another discovery.