My boat has that, works great.

It took a little tinkering with the lengths but we got this set up on our boat under the tramp and its been great. Works especially well if you have the retrieval line running through a ring at the clew to pull the sheets into the snuffer when you douse. No extra slack in the sheets, no extra slack in the halyard.

My halyard is under the tramp.

My tackline runs under the tramp to a back to back block, then is tied off at the front beam.

That back to back block has a bungee running through it. The bungee goes through a couple of little harken blocks tired off to each corner at the rear beam. Both ends of that bungee have a block on it that the halyard runs through. I want to get rid of these blocks and switch to rings to keep asshholes from ruining my day.
The block tied off to the starboard side of the rear beam is larger because the bungee runs through it twice.

It sounds super complicated, but once you see it in person is really simple and it has a 2:1 to a 2:1 on the take up, so you've got miles of slack with kite up, but still pulls it up tight when snuffed. I haven't noticed any difference in effort to snuff the spinnaker either.

Hey Karl, have you seen the ronstan shock blocks? Absolutely ideal for spinnaker halyard take up! The rope runs a little smoother through them.

I have. Hadn't thought about using them there though.

This is a vid from Rich Vilvens Capricorn, that is a pretty tricky under the tramp halyard! looks like it works pretty well too.

As a quick follow-up on this. I decided to give it a go and stick with the original double-tapered design based on 5mm Robline Dinghy control. I have an undertramp takeup that works well and didn't want to risk the chance messing up the delicate balance.

Turns out my concerns on doing this myself were completely unfounded. This YouTube video was hugely helpful.

https://www.youtube.com/watch?v=7bWZpUQZO8E#t=190

Hardest part was digging the effing core out of the plait. Had to borrow my wife's reading glasses just to see what the heck I was doing. I think it took all of a couple hours beginning to end. Next time would be half that.

Now its on to making a new set of dyneema trap lines...

I'm stupid. I pull the entire core out, and stuff in a different color, just because I'm a douche, and like having things that aren't off the shelf. But yeah, it takes some patience to pull the dyneema through that tight weave.

The more you do it, the better you get. I could make a halyard in 15 minutes at one point. I haven't had to do it in at least a year and a half, but my spare is gone, on the boat, and it's looking tired, so time to make a couple more for myself. I'm sure it'll go swimmingly....

Start buying line by the spool.

I could make them in about 8 minutes each of I was making 5 or more at a time. Not any more though

It is easier to use a 5mm single braid like Robline Racing sheet or Mafioli Swiftcord spliced into 3mm dyneema, which is also single braid. Much easier and faster. I hate trying to splice those tightly woven covers.

I remember you mentioned it being tough to work with. Now I'm a believer. Not sure I'll try that tight braid again. Picking the plates out of the cover to taper it was maddening.

Hmm, I didn't hear anyone mention splicing in a soft shackle to the head of the halyard? I've done that on my last two halyards and its worked great! Very clean. I do the soft shackle first and then strip the core out of the cover to make sure I get the length correct and then cut off the extra cover to provide the tail for the kite.

Here is the youtube link for the integrated softshackle:
https://www.youtube.com/watch?v=rRKOkYqWpug