[I didn't originally intend to post this, but then as people came over I pressed Publish and so there it is. Why not remove it with my Blogger Omnipotence? Cuz I'm a silly boy. Now I can explain why I wrote it: over at Jimmy Akin's blog, he discurses some fancy new find in Egypt and then a commentor asked about its potential impact on the biblical canon. Which got me thinking....]
Pardon any apparent casuistry, but in a very nitty gritty sense the canon is not closed. Insofar as we are not certain exactly which *copies* of the canonical works to use (copyist errors, textual variations, lexical ambiguity, etc.), we must admit a certain fuzziness around the edges of any dogmatic decree of canonicity.
It seems to me the Church sensibly avoids this "problem" by relying on a realist metaphysic to establish the canon, as a collection of instantiations of the inspired autographs themselves. What would be needed to absolutely hermetically seal the canon is a more precise (but basically unworkable) trope metaphysic, whereby THIS or THAT unique instance of said canonical work is THE canonical MS. Realism says X instantiates or actualizes the idea of "red." Nominalism says "red" actually just means the (socially/ statistically) accepted set of all things said to be red.
Trope theory, by contrast, tries to speak between them both by saying there is such a thing as "redness" but that it is "THIS red thing's redness." Thus, there is a supersensible reality which red objects really do instantiate ("redness") but which is itself based on BEING instantiated by *particular* red things. Redness, then would be what all red things truly instantiate, and not merely what we consider the set of all things to add up to. And yet, each thing's unique instantiation of redness in turn depends on the reality of redness as such. X's particular redness cannot be red without redness, but redness is not red without "accepting" X's particular redness "as its own."
By extension, there are numerous (?) copies of St. Paul's letter to Titus, but the epistle itself only comes to us by way of the concrete MSS. we have. Are we lacking the "real" epistle if we only have later and minutely varied copies? No, because each copy of Titus is what it is precisely by virtue of really and truly instantiating the "ur-epistle" itself.
These are abstruse considerations, I know, so I leave it to my canon-law and dogmatic betters to explain whether the canon, as a set of instantiated but non-tropically defined works, is unalterable. I suspect so. I also sense there is something useful here in a consideration of Scripture and Tradition… perhaps how the same message may be instantiated in its own unique mode….
But for now we just need to get down to finding those golden tropes!
 As much as I blush to admit it, I really do love metaphysical conundra. In small measures. About the right topics.