Yes, it is. Tor Browser already does this: https://www.bleepstatic.com/content/posts/2017/10/30/CanvasF...
That specific wording may be a touch too verbose for the average end user, but it's not impossible nor is it strange. Just include a note about how this is 99% likely a fingerprinting measure; option b) isn't so bad in this case. Of course, due to the nature of how fingerprinting works, the absolute breadth of features that would be gated behind something like this would be offputting.
I am also wary of what you suggested with gating this kind of fingerprinting to when the website has positively identified the user anyway; in a way, this seems to me even more valuable than fingerprint data without an associated "strong" identity.
