Just a suggestion...

I am not an english native, but maybe you could use that ? It might be completely "anachronique" though... Or use something like "you're full of dejections"

Could probably do, adding a new color to the language! French is richer than you think, by simply adding a "aille" to the end of scatologic words!
Cf: "La rue au Moyen Âge", Jena-Pierre Leguay. Editions Ouest France.Thelanguage of "Les Visiteurs" will sound very cultural! ;-)

nice old English word

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Note added at 35 mins (2006-02-02 09:54:15 GMT)
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http://www.etymonline.com/index.php?term=dung

perhaps

bag of shit, full of shit etc.

Scientific Approach :))
Scholars trace the word back to Old Norse origin (skîta), and it is virtually certain that it was used in some form by preliterate Germanic tribes at the time of the Roman Empire. It was originally adopted into Old English as scitte, eventually morphing into Middle English schîtte.
The variant form shite is found in many regional and social dialects, especially in Scotland, Ireland and Northern England where it is generally used as an adjective, and is sometimes used in other parts of the world as a less-offensive (at least in intent) form of the word "shit".
The word shit is used by English speakers, but it is considered an obscenity, and thus is usually avoided in formal speech. A less vulgar substitute is crap, which while still impolite and/or emphatic, is not considered obscene.
I think you don't need the Middle English term in this case...but the obscene one:)

Is he saying they are shit (no moral fibre? Eat Allbran!), or that they are only flesh and blood (no supernatural forces)?

In the latter case, I rather like "you are (thou arst) but/nothing more than entrails and (stinking) excrement", even if, strictly speaking, excrement is not exrement until it has been evacuated from the entrails. Adding an adjective or two like "fetid/stinking" might reinforce the insulting nature ...

I like entrails (the word). Seems medieval to me. Goes with hanging, drawing, and quartering. The French have it with a shallot sauce.

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Note added at 34 mins (2006-02-02 09:53:16 GMT)
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What I mean is, "tout comme moi" makes it sound more like a philosophical statement than an insult.

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Note added at 2 hrs (2006-02-02 11:56:41 GMT)
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Re. your note : that's what I refer to as "philosophical".

Monty Python (How do you know he's the King? Coz he's not covered in shit like us) nothwithstanding, "shit" does not strike me as typically medieval, though it (the word as well) surely existed. SOED gives first recorded uses of "shit" as 1585 (excrement) and 1508 (contemptuous epithet), though that appears to be chronologically backwards!

I don't see your context as insulting; crude, but not insulting.

Another idea - maybe transfer the "medieval" sounding terms to the subject + verb...

All of the entries sound good. I found this interesting site, which includes the use of terms according to Medieval usage (see link below).

"tripes et merdaille" - guts and scum (possible context)