Wikipedia’s entry on the Cento has some more information:
The rules for the Cento were written down by a Roman poet named Decimus Magnus Ausonius (circa 310-395).
The poem may be derived from work by the same poet, or from several. It appears one popular challenge was to rewrite Bible verses (Old and New Testament) using Virgil’s Aeneid. Verses (lines of poetry) may be used either in their entirety, or divided in two; one half to be connected with another half from elsewhere. Two consecutive verses should never be used, nor should less than half a verse.
Cento also works with prose. The recent controversy with the Havard student’s book comes to mind, but whether her actions were intentional or not, they differed from a cento in three major ways. 1) She made changes to the words. The changes weren’t significant enough to counteract plagiarism charges, but they were still changes. 2) She added original material between the sections of copied material. At least, I assume she did. Cento uses only the words from the original sources. You can not add your own. 3) She didn’t credit the sources. That’s what makes cento and found poetry legitimate — the author admits to what they are doing, and gives credit where credit is due. The Fair Use provision of the US Copyright Law (and likely similar provisions in International Copyright Law) allows for usage of copyrighted material within some guidelines. But you can still end up in court arguing with an author over whether you crossed the fair use boundaries, so it’s safer to use public domain sources.
Interpreting Ausonius’s rules for prose, it seems to me appropriate to replace a verse with a sentence. In instances where a full sentence isn’t used, it seems appropriate to insist upon a full clause, whether it be dependent or independent. But this is my interpretation.
Here’s a brief section of dialogue I stitched together over the past hour (it uses material that is definitely still under copyright, and if JKR or any of her publishers ask me to remove this from my website, I will do so, but I do feel it falls within fair usage.)
“Now! (SS p. 9)” Ron’s flask exploded. (HBP p. 515) Harry gave a hollow laugh. (PoA p. 78) “Excellent!” (GoF p. 270)
The clanging doorbell rang again. (OoP p. 106) Harry stared at it. (HBP p. 339)
The office door opened. (CoS P. 207) It was Quirrell. (SS p. 288)
Harry’s mind had gone blank with shock. (PoA p. 332) He strode across the room towards the stairs; he half expected Ron to stop him, he would even have liked Ron to throw a punch at him, but Ron just stood there in his too-small pajamas. (GoF p. 336)
I don’t claim the above is perfect. I spent an hour on it, and it’s not an easy task. This illustrates how beneficial it might be with prose to relax one of the rules of Cento and allow for changing names. While I feel the progression of the sentences above work, they could be slightly confusing if you are familiar with the characters from the original.
SS = Harry Potter and the Sorcerer’s Stone
CoS = HP and the Chamber of Secrets
PoA = HP and the Prisoner of Azkaban
GoF = HP and the Goblet of Fire
OoP = HP and the Order of the Phoenix
HBP = HP and the Half Blood Prince