Centre of effort: Wouldn't the pointy sloop CE, yawl CE, and junk CE all be the same given the same hull?
In reality, there is actually no such thing as centre of effort (and also no such thing as centre of lateral resistance).
Both are moving targets, quite variable as the hull heels and pitches, and sails flex and billow and "angle of attack" constantly changes. They exist only in theory.
The convenient reference points which are used by designers, because they are static and simple to calculate, are usually referred to as CE and CLR but in fact they are merely the geometric centres of the sail and the hull profile.
There are rules of thumb as to how these two geometric points should relate to each other, [edit: the "lead"] but it is a bit of a magic art and I think experience on the part of the designer, with the type of hull in question, is what most likely will give the best (or optimal) relationship for most of the time. [edit: with some hulls the equations change markedly when the boat heels, so there is no "lead" which is going to be correct all the time - sometimes, with SOME boats, you just have to live with a bit of lee helm in light airs, and too much weather helm when the boat is heeling a lot. Or reef early.]
All of the articles I have read suggest the centre of area on a junk rig should be slightly aft of that of a BM sail. I have noticed that too, so it probably means something.
However, for the record (I am speaking only of the SJR) Slieve, who created the rig, reasoned that in the case of a conversion from bermudan rig to SJR, the SJR should be able to have its centre of area pretty much exactly where the centre of area of the designed bermudan rig would go. So far as I know that has proved satisfactory in all cases, and it proved satisfactory on mine.
For the "contiguous" or "unsplit" junk rigs, Len is probably right. In any case, its only ever an educated guess, and I would follow the advice of the designers of those rigs.
As for Dean's suggestion of sailing on mizzen alone - I think Jan is gently pointing out that sailing on mizzen alone is hardly likely to be possible. Perhaps Dean was meaning "under mizzen and staysail alone" ie without the main. But with a junk mainsail, I am sure Jan's suggestion would be better.