TD, that's interesting : the overlapping cinch is what the Chilean saddles use. There's a normal one at about 7/8, and then there's a surcingle girth that goes over the seat and overlaps the cinch underneath. Actually I have to say it kind of annoys me, there's a lump of latigo underneath the knee, or two buckles : either way, it's right where you don't really want it. But you get used to it.
The double cinch that is on Minina, I see that it's something the Canadian packers use a lot. I think it's adapted to the configuration of your horse - if s/he's got a nice hay belly (which is what I want in my trip horses) then it'll hold on just fine. There's a strap between the two to stop the rear cinch slipping back into her groin. Canelo (the draft cross) doesn't like the double cinch - see my story about the buck-fest . I still want him to be able to do it, though, so we're working on lots of desensitisation. But Minina, with sacking out and lots of preparation accepts that more readily than she likes having a tight cinch. She takes a load with it and it doesn't bother her. So I just went with what they seemed to prefer.
RSS, the cavesson with a ring on it is interesting. I was also wondering about rigging a 'triangle' of either rope or bungee cord between the two side rings of the headcollar so that the lead rope attaches to the point of the triangle. That way the headcollar doesn't twist round according to what side the horse is on - but if they're not leaning on the halter, that doesn't happen anyway. Hohum. Your cavesson idea is neater.
It's funny, because I was thinking about trying the Micklem bridle on that riding horse that I was talking about, and there's a ring on the cavesson for lunging purposes...
Re. The rig, we'll have to see for the stability. We tried both saddles on both horses, and although Canelo looks longer than Minina, the purchased saddle bridged between the back of his shoulder and his ribs, so was an automatic no-go. Whereas the other saddle bars fit him almost as though they were made for him... I'll let you know!