Barn Jams were pretty much just that, jams. Sometimes there was a pre-existing idea that was jammed over (such as Island jam). Some of the more 'structured' ones got the odd overdub, for instance no. 4 in this set has the slide guitar as an overdub. Phil was there cataloging everything, as is his wont!
I did actually mix the Barn Jams in surround, albeit that it'd not be that spectacular, just a 'stretch out' (horseshoe shaped stereo if you like). Not enough room to fit the extra audio data on the DVD version though, and the powers that be weren't willing to have the BD & the DVD different. You haven't missed much TBH
Ron: as you can see, no Nick. Basically it was all Stevie except for some later bits, where Andy did it, as flying Stevie over for one day is a bit excessive! We ended up with "Dancing right in front of me" being a montage of the two of them, you can work it out if you think about it, the pitch on the snare drum is really different. The exception is "Girl in a yellow dress" which is Martin France, this was actually recorded 10 years ago during the On an Island sessions.