: while i wouldn't advocate this for anybody as i have not see a yak with it,

Betsie Bay boats come with deck rigging that only runs from just behind the cockpit to near the front bulkhead. My Nordkapp and Pintail had full perimeter rigging that ran beside the cockpit. I believe that most VCP boats are rigged this way from the factory.

: i think lines past (or ability to place past) the cockpit make as much sense
: as anywhere as long as they are out of the way.

Therein lies the problem. Unless the boat is quite wide, the lines will constantly get in the way when attaching a spray skirt. On the boats above, one of the first things I did was to get rid of the rigging beside the cockpit.

: if easy unlatchable

I really dislike the idea of anything that would introduce slack into the deck rigging, especially in a rescue scenario. For this to work, you would need to have the rigging tied off a the attachment points for the cockpit rigging. It's also difficult to see how this could be done without the rigging next to the cockpit being slack.

: maybe would make a good leash,

With the attachment point so close to the center of the boat, you would never be able keep it pointed forward.

: short tow,

this might work

: remount sling,

I tend to view theses as useless gimmicks that tend to capsize the boat and put a lot a stress on a paddle float rescue system. Learing proper technique is a better solution.

: paddle harness

Perhaps when resting, but not if you're going to be paddling. There are better ways of securing a paddle.

: or temp sidefloat mount.

I doubt that it would make much difference if you mounted it a few inches forward or aft of the cockpit.

: to me seems a most useful location not yet exploited.

Try it sometime and you may find that it's not nearly as useful as you think.

One thing that I've seen that does seem useful is built-in, recessed hand rails beside/behind the cockpit. Very few boats have these and AFAIK, none of them are built - or even available - in the US. It could be done on a wood boat, but I haven't seen it.