But there is a tradition or convention established just by practice — what writers have a tendency to do, and thus what readers also will have a tendency to do on the other end of the process.

That’s just a quasi-statistical observation, and I think that’s what the phrasing “doesn’t usually go mid-syllable” implies. Are you suggesting that the cutoff does usually go mid-syllable?

I can see a reply of “No, I’m suggesting it would usually go wherever the speaker actually cut off”. But (1) we’re not privy to that, all we have is the printed result, not a window into the (fictional or real) speaker’s intent; (2) in a way that claim is in a sense necessarily true, and hence vacuous — of course it’s (at least supposed to be) wherever the speaker would cut off — but that then defers to a more concrete empirical or quasi-empirical question, in fact (or in representation) where do speakers tend to break off, mid-syllable or on syllable boundaries? And at least in representations, it is overwhelmingly guided by the syllable-boundary convention.

On the other hand, I strongly suspect you and others are absolutely right that it was Jacques’s intention to suggest the completion would be “–ck”. The quibbles from me, and others before me here, are meant to indicate how difficult and awkward it is for him to try to get that across.

The difficulty is half because of the generalization r convention that we’ve been belaboring here. And half because of English spelling and pronunciation patterns, so that the spelling “co-” is pretty ambiguous about presumed pronunciation. It could be the COH of “co-operative”, especially because of the pressure to make that a syllable on its own, or COH as in “coal” if we accept mid-syllable break. Or it could be the CAH of “co–ck”. Or the r-colored CAW or CAWR of “cord”. Or the CUH of “come”. Or the CUU of “cook”. Or the CUUW of “coop”. So it’s only our dirty minds that make his attempt half-work or more.

Doesn’t it go wherever the speaker got cut off?

But Mitch4 is right. That is what kept me from seeing it. The only word I could
come up with was co-processor which didn’t make sense. Padraig does have
a good answer.