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I have a short-scale Classic Player Jaguar Special HH with blendable HB's. What are some humbucker-sized pickups that can cop a Fender Jazzmaster tone when split (or blended all the way to the single-coil side)?

No such thing. Sorry about that.

Surely, there must be one HB that sounds *more* like a Jazzmaster pup when split than others.

Yeah, the SD P-Rails. The p-90 bit of this sounds a little like a jazzmaster pickup, but not really.

To my ears, suspect as they may be, a reissue wide range humbucker sounds an awful lot like a Duncan QP for jazzmaster. DO AN JAGBLASTER.

The reason a Jazzmaster pickup sounds the way it does:
-wide, flat coil
-pole pick up magnets (traditional single coil construction)

Most humbuckers use a bar magnet as opposed to the pole pieces, and are not as wide or flat.

I guess a low powered humbucker-sized P90 might get in the general vicinity (at least closer than any humbucker), but nothing will really quite do it like the real deal.

However, I do think Curtis Novak makes something that might work okay.. http://curtisnovak.com/pickups/hum-WR.shtml

Also, if you're looking to make your Jag sound like a Jazzmaster, it's going to be quite a bit different with the scale length difference... though you are mostly there with the bridge/trem construction.

Do an P-Rails

I might put them in my Classic Player HH someday

As is I can't be bothered

generally, split humbuckers sound weak and thin, not really anything like a fat single-coil like the jazzmaster pickup.

although, a high output type humbucker might sound a bit thicker when split. something around 16 ohms or higher.

I would also recommend a P90 size humby. dream/mean 90, Phat Cat, or P-Rails.