Aussie Saddles from Kates.net

I would get a saddle custom made with a wither tracing, I think Kate's might offer this, there is another Aussie saddle sight out there that does but I can't remember which one it is.  Haffies are soo hard to fit saddles to, I have 3 of them and none of the 2 have the exact same saddle fit but all 3 of them do not fit traditional ENglish or Western saddles. 

I have enjoyed riding in Aussie saddles and feel they are more forgiving in fit than the traditional Western style.  Someday I will have a Syd Hill Aussie saddle custom made for Sweet P.

Yes, Kates carries the Rideabout and Syd Hill brands, both offer custom fits with wither tracing, the Rideabout is reasonably priced at $600.

I called her yesterday and she was not sure she could fit my ponies.  She had me send a wither tracing, picture, etc.  Her concern was that the saddle might be too long for them.  That is what we are running into with western saddles---anything with a wider tree only comes in a 16 or larger seat size.
 
The English saddle we have seems to fit the Haflinger pony pretty well.  It is short enough and has contact all along his back.  We just need to make sure it isn't pinching when my daughter rides.
 
I wish saddle makers would realize that there are LOTS of wider ponies out there with riders that need 12-14 inch seats.  We need nice saddles on full QH bars but shorter and with smaller seats.

tryhttp://www.montanamountainhorse.com/
 they have several draft trees and can shorten the tree for you.  I had them make Sweet P's saddle.  You may try a Friesian tree for swaybacked horse.

Ottakee-

I have been thinking hard on your question, and it seems to me that the traditional serge panel style Australian saddle is actually the best solution for your horse.  Unfortunately, many of the modern Aussie saddles have a felt panel with a wider bar (much like a Western saddle).  The advantage of the serge panel is it is 'stuffed' and will adjust to your horse's back; also the bar length is much shorter.  here is a good explanation of the two:
http://www.downunderweb.com/sydhillcustom.htm

The best advantage of an Aussie or an English saddle for a shortbacked horse is the excellent spine clearance in the chamber (gullet).  The horse has a natural rise towards the loin, and saddles that are too long generally tend to rub the spine in this area.  Having the short bar and deep chamber prevents this, even if the saddle runs to the loin.

You may also consider a dressage saddle, they are quite comfortable for trail riding and are just a tad shorter in the tree than the Aussie stock saddles.

Regardless of your decision, you should invest the time and money in working with a company who will customize the saddle for your horse.  You may also need to consider using a special pad with shims for your swaybacked horse, there are some very nice ones on the English tack market that use gel shims, these would work under an Aussie saddle.