I haven't sat in a prestige eventer but I just bought an Ideal Impala for my flat backed wb and it fits him beautifully. I had tried a black country on him (a wexford) and it also fit him great but I found the twist too wide and it wasn't quite right.
On second thought, I might have ridden in an old prestige eventer (not 100% sure). It didn't work for my thigh but I understand the tightness feeling and my impala is definitely secure. If the Stubben Roxanne wasn't such a curvy tree I would suggest that.
assuming the prestige eventer rides like the nona garson - narrow twist, forward balance point- the french saddles and amerigo will ride like that.
I used to have a flat Butet (which I freakin loved and still miss) which worked very well for flat backs. I have no idea about what your looking for security wise, but most Butets have blocks etc (mine didn't, still miss my cushy pancake). Deep seat Butets are curvier and may not work for your guy.
I like more of a French style ride, and I disliked the Black Countrys because of how they rode.
I fall on my head in anything but an Eventer. LOVE LOVE LOVE LOVE.
I catch rode my friend's horse last year and she has one of the Pessoa Cross Country saddles with the exchange system.
I was very comfy in it...and it wasn't even a long flap. (my Prestige is long flap)
My friend bought her's through chicks discount I think. She found it on ebay and asked if Chick's would match the price and they did.
Often Dover has them for sale when they send out the closeout catalog. I've even seen a long flap! for $999.99. Next time I see one I'm going to buy it just to have--since I buy and sell horses. Good saddle to have in case my Prestige doesn't fit the mount of the month.