I don't think I could wrap my head around that one even if I had had a cup of coffee already this morning.
My recollection, from the brief time I had a spey rod and played with it, is that the leader should be roughly the length of the rod. So you are talking something on the order of 12' to 15' including a short tippet. Uless you have a really long board / jig, you will have to do it in two parts. There is a way to a make loop to loop connection between two parts of a furled leader which a couple of us have used on long Tenkara line / leaders. Let me know if you want to discuss that part of it.
Spey lines are pretty heavy duty things compared to conventional fly lines, and the size designations are quite different. I think you can figure making a spey leader about two size designations bigger than you would for a regular fly rod - a 6-7 wt spey is more like an 8-9 fly rod wt. If you get up to an 8-9 wt spey, you are talking about a really big leader. Two things - you're going to have use large size materials or use a whole bunch of strands of fly rod leader type materials, and you probably won't need to reinforce the leader with braid since it will probably lift a horse or two without the braid. Well, maybe a calf.
The leader is key to spey casting since it is what makes the anchor for the D loop and ennables the cast. Since this is not particularly subtle type casting, at least as to what is happening close in before the cast is actually made, I don't think you have to worry about how supple the leader is. In fact, I suspect that suppleness may actually be a disadvantage in this situation. ( Gasp - did I actually say that.
Where I'm going with this, as I get a headache from my first time ever thinking about the monster you are creating, is that I would start with a pretty heavy ( but still supple ) mono and I would be thinking in terms of a configuration like 14 strands in the butt secton, 12 strands in the mid section, and 8-10 strands in the tip section. For proportions, I would start with about 2-3' each for both the tip and mid sections, and the rest would be butt section.
I think the above would be a starting point, conceptually, and a base to work from once you know the particulars of the line weight you are actually building the leader for, and then you can start refining based on experimenting by casting with a spey rod set up.
P.S. Here's another idea - call John Harder at Rio in Idaho Falls and see if he is agreeable to Simon Gawesworth talking with you about spey rods, lines, and leaders. I think Simon probably would make a little time for you, but I also think you need to clear it with John. Tell John I pointed you at him, and give him my best if you do talk to him.
Now I'm going to go have that cup of coffee, and then go fishing.