Looking at the Op's profile, Ric clearly knows dogs, and therefore already clearly knows the answer to his own question yes, dogs clearly possess the requisite awareness/consciousness/whatever for "consent". No need to grossly over-intellectualize it.
What's really, really clear is that dogs can express enthusiasm, and conversely reluctance. Consent is maybe a bit more subtle, but it's not a question of if the dog can express it, but of whether or not you know the dog well enough to read its expression and body language. I've been in plenty of dog situations where it's clearly communicating, "Yeah, yeah, okay, but it ain't my first choice."
How do I know for sure that's what the dog's feeling/communicating? Well, there's the https://en.wikipedia.org/wiki/Turing_test for intelligence, whereby Turing pretty much gave up on a quantitative definition/test for "human intelligence/consciousness". And I'd imagine we're pretty much barking up the same tree (sorry, couldn't resist) for "dog consent". And they most unquestionably pass my Turing test for that.