Greater deamon looks good already.
I like the peeled flesh look on him.
Greenstuff should normally work on larger surfaces as it cures rather slowly.
Thing is you'd need so much of it.
You used paper and glue for a base to sculpt on? Based on a wire mesh frame I guess.
I was personally thinking of using modelling clay, like fimo, for the basic shape, then detail with greenstuff.
You could give that a try for the head.
(the confrontation minis were sculpted mostly out of fimo, it serves it's purpose rather well)
That's going to be one big bugger.
Ever thought of useing the Tyranid monstrous creature design rules published in WD and (oh darn! just forgot the name of the 40k supplement books) to make a set of rules for friendly games?
The 1st edition plaguebearers do indeed not look as spanky as the 2nd edition ones.
With 60 of the buggers though I wanted some extra diversity
Plague bearer madness doesn't even cover it really