I do agree with the comments on my last post: if the characters, dialogue and plot are good enough, then period trappings shouldn't matter.
I wonder if the problem is rather more one of perception: Susanna Forrest says "If publishers are prepared to take a gamble and let pony books escape from the "posh gel" stereotype...", and I wonder if this is it. Ponies are seen as something beyond the reach of most people and if you stock a pony book you are in some way promoting that difference. That might be why it's the Heartland and the baby pony fantasies which are the ones stocked. Heartland is set in the USA, and therefore has the exoticism of abroad (albeit an abroad we're very familiar with) and you can excuse a lot of PC sins by combining them with fantasy - like Harry Potter and the boarding school.
Maybe that's why the Jordan books are so pink and sparkly - it's the publishers' way of saying "This isn't real life: it's just like Barbie - make believe."
I think this is another view that does children a disservice. Children don't generally think "Oh how ground down and miserable I feel because they go to boarding school and I don't." I was state-school educated and when I read the St Clare's stories I loved them and didn't think for a moment how dreadful it was that they had a completely different lifestyle to mine. (When my parents suggested I do a scholarship exam for a local public school I resisted so strenuously they gave in.) Children tend to read things for other reasons than to compare lifestyles, because generally other things are important to them, like good triumphing, the baddie being confounded or redeemed and so on.
I don't think the modern pony book has to include all sorts of today's issues: it needs to be recognisable , yes, but not necessarily be chock full of issues. Gillian Baxter's Bargain Horses was I think an excellent example of the right way to go, and yes, K M Peyton's Blind Beauty is a corker. When I read it, it was one of those books that I was so absorbed by that meals went uncooked... But it's good not because of its background, but because the author makes you care what happens.