Hi, I know this post is pretty old, but I'm also looking at an Abetta endurance saddle for my arab cross. She's an arab/quarter horse cross, but her build to me looks very arab. She's only 14.2 hh and she does have a pretty short back.
Unfortunately there aren't any tack shops in my area that stock Abetta (or anything other than $3000 'performance' saddles) and so I'll need to order everything online. I've already ordered one saddle I need to send back (an Australian stock saddle - thought I'd try something new, and I hate it :( ) and I'd really hate to send back another... but if that's what happens I guess that's what happens.
Anyway, she does have a slender back and I think where you're referring to the saddle filling in 'hollows' would actually work for her... I guess I'm going to bite the bullet and order one. I really hope I don't have to send another one back!
Did you end up getting a saddle, jillybean? What kind?