Yes, of course it can go well. But why compare your horse to a starved rescue horse? That's one extreme situation, and not one you should ever put a pregnant horse in, even if it might go well.
You should increase the feed before you start seeing ribs, you should only be able to feel the ribs on a 8-9 month pregnant horse.
And just because some mares give birth to healthy foals, while looking like a picked fence, it's not OK to let it happen.
I'm not trying to be rude or anything, but we had a few persons around here that said the same. The mare looked "ribby" when she was 8-9 months and the owners only then realized that they should increase the feed, but she didn't put on weight fast enough. She stopped eating three weeks before she dropped her baby.
She never recovered, even though the owners fed her so she had hay to eat 24/7 and as much alfalfa and dodson and horrell feed as she possible could eat.... and had to be put down. She was healthy otherwise, and the vet came out. Nothing wrong with her, she just wouldn't eat, because the she thought the baby took too much space in her belly.
I wouldn't want anyone to go through that. Ever.