Hey mate. If I'm in the country then, I'll do it with you! You could take out a free classified ad in the back of horsedeals, and on various other horsemags.
There's an american trail riders magazine that does heaps of this kind of long-distance trekking (Can't remember what it's called, but I bought it from the newsagency in the city near cnr Collins St & William St, next to maccas).
Saddle - a really well fitting one!! You'll want to pre-book saddle fitters, farriers & vets along the route. If it were me, I'd look at a treeless one (but I want one anyway!). But you'd want something as light as you can get it, but one that you can attach a fair amount of stuff to - there's a Wintec Endurance saddle. It has lots of D rings, etc. You may have to design some kind of over-sized saddle bags too.
Horses - If it were me going on my own, I would take two. One to ride & one to carry the stuff. But interchange them. So you might also consider a pack-saddle. If you call one of the Alpine trail riding outfits, I think they use pack-saddles (I've seen photos on one of the websites). Along with this, you'll need to practise precise packing strategies to ensure that both sides of the pack-saddle weigh the same. I have seen an Aussie site with stuff about this, but can't remember where.
If you were going with two ppl, you'll have to assess between each other how light you can pack & see if you can get away with 3 horses between you.
Distance per day - There was a huge trail in Queensland that went from somewhere up north down to Brissie... I'll see if I can find the link. I saw part of a doco on them. They had one crazy woman that walked the whole way. She'd leave every morning before the horses & get to each camp after them! NUTS!! You could drop in and out of the trail where you wanted, but all the people that had done it from start to finish rode into town in red blazers. It was cool. They had all up nearly 1000 riders from memory. Only about 10 did the whole thing. Found the website...
Sponsorship - A group here in WA rode from ??? to Kalgoorlie to raise money for breast cancer research. There was about 10 riders. I don't know if they got 'sponsorship' in the form of cash, or rather they organsied people to donate feed & things like that at each town. I'll see if I can track it down. It was in the local paper a few months ago. They also collected funds along the way & organised with local community groups to hold BBQs when they rode into town.
Other info - you might consider support vehicles? Especially for more remote places where water would be difficult to get. You'll need collaspable buckets too - you know the plastic ones that I've wanted forever (for not other reason than they're cool!) I've seen them for sale somewhere recently.
Alpine riding in Vic... which trails you can use & some advice (such as in many places, u can't use hay, only processed hard feeds, to control the spread of weeds)
A place to buy your long-distance tack...
Someone's own account of part of the trail ridden...
Sponsorship/fundraising - a chick rode 250km for the Fred Hollows foundation.