I certainly haven't seen any evidence beyond what folks have already posted here. I think we can make a few reasonable deductions, though, based on internal evidence.
In Kaplan's Encyclopedia, Vol. 3, he shows an illustration by Smith. The caption reads, "'A Figure of Awful Beauty.' The angel from the story 'The Garden' by G.J. guards with a fiery sword the Tree of Life, which lays at the northmost point of the earth. The flaming hair resembles the hair of the angel on the Lovers card of the Rider-Waite Tarot. The Green Sheaf, issue 10, 1904." The illustration shows a mountaintop covered with flames, out of which rises a huge, transparent figure with both arms upraised, one hand carrying a wavy sword. Beside that illustration, Kaplan places the Lovers card, and it is indeed quite apparent that Smith drew on the earlier picture as inspiration for the angel on the Lovers card. To me, this suggests fairly strongly that Waite did not "spoon-feed" every aspect of at least the Lovers card, since the major influence on the angel's appearance was a previous illustration by Smith.
In a similar example, Kaplan places side by side the Four of Swords card with an illustration from A Broad Sheet, 1902, on which a woman reclines in a church or mausoleum in a very similar manner to the knight on the Four of Swords. To me, this indicates that Smith used her own judgement to arrive at a picture to illustrate the Four of Swords, although admittedly this conclusion requires a little more speculation than the Lovers example.
Then, there is the similarity between several of Smith's cards and cards from the Sola-Busca deck. The similarities are so great that it's obvious that Smith used the Sola Busca cards as inspiration. Since the Sola-Busca deck was only available to be seen in a single set of photographs at the British Museum (I can't remember where I read this but I do clearly recall reading it somewhere), it's reasonable to assume that Waite directed Smith to go to the museum and look at the cards. From what we know about both Waite and Smith, I think it's likely that Waite would have known about this deck while Smith probably wouldn't have.
Then there's the way Waite writes about the numbered Minors in his book. He seems sometimes at a loss, sometimes saying a card could mean such-and-such or it could mean its opposite, or commenting that the picture doesn't reconcile this meaning with that meaning. He writes like someone who has been handed a bunch of pictures and who now must make sense of them, not like someone who decided what images the cards would show.
My inference from this (admittedly speculation on my part) is that he did not dictate the specific images for the numbered Minors, but rather handed Smith a collection of divinatory meanings, directed her to go to the British Museum to see the Sola Busca to get an idea of the kinds of pictures Waite had in mind, and then let her come up with the pictures on her own.
I was already familiar with the passage quoted by Vincent in which Waite says he "spoon-fed" Smith, but it wasn't until reading Vincent's post that I noticed for the first time that although Waite says the Trumps were produced under his supervision, he only says he "spoon-fed" her on three specific cards: the High Priestess, the Fool, and the Hanged Man. This leaves us with the suggestion that perhaps Smith had more leeway with the other Majors, as she obviously did with the Lovers.
Also, it's interesting that Waite says "...If anyone feels drawn in these days to the consideration of Tarot symbolism they will do well to select the Trumps Major produced under my supervision by Miss Pamela Coleman Smith." Note that he singles out the Majors. This might suggest, as I posited above, that Waite specifically directed Smith on the Majors but gave her much more leeway on the Minors, as Vincent suggests.
I love to speculate on these things, especially since we'll probably never know for sure!
-- Lee