"What are the advantages/disadvantages of a 12-fret version of, say, an 812ce?"
Well you basically nailed it, a 812 size is more compact, then add to that the shorter scale neck and then the even shorter 12 fret neck and you've gained over an inch where your left hand will be closer to you. The grand concert 12 fret is a really comfortable couch guitar.
As for fingerstyle, I think there's more dynamic range with a 12 fret, you can play softly without sounding thin or as loud as you like. Then there's the extra depth of tone you get from all the wound strings. Dropping the bridge to the sweet spot gives more volume as well.
This doesn't get mentioned much, but if you play with a capo, your tone is not going to thin out as much playing a capo on a 12 fret because you have more depth of tone to begin with.
A disadvantage is if you're so used to Taylors normal bright tone you can't get used to how their 12 fret sounds. I really wouldn't buy one you couldn't try out or return. There's a distinct difference in tone between a 14 and 12 fret and I've gotten to the point where I don't really enjoy a 14 fret anymore, my 812 just doesn't get much use now.
A possible disadvantage to 12 frets is a possible loss of clarity since you're now loaded up with more mid-range and bass tones. IMHO, Taylor's first 12 Fret model, which I once owned, suffered a loss of brightness that I think they've fixed now with improved bracing. A spruce top cures this brightness deficit as would brighter sounding 80/20s, although the warmer phos/bronze elixars are my favorites.
As a side note, with a mahogany topped 12 Fret, I've found that going to lighter gauges for the bass E and A string really helps improve the clarity of all the strings. Similar in idea to Taylor's new HD gauges, it seems that a bit less string tension from the 2 bass strings gives the higher strings a bit more definition and a better balance overall. I did that with a all mahogany 512 I was about to give up on and it's a keeper now.
I'd say give the 812 12 fret a try if you can find one, you'll know in the first few minutes if you like it or not.