"saddle-fast" sounds Denglish to me, especially in its non-equine, figurative meaning. Granted, it's in the OED, but even there only the literal meaning is given:http://www.oxforddictionaries.com/definition/...
Definition of saddle-fast in English:
Firmly seated in the saddle.
Early 19th cent.; earliest use found in Walter Scott (1771–1832), poet and novelist.
I can't vouch for BE, but I have never come across "saddle-fast" in AE. I'm not an expert in equine matters, so I don't know if American horse lovers actually use "saddle-fast," but the only online examples I found were in bad translations on German/Austrian websites. And no, I didn't check every single hit, only the first 80 or so of about 8,000 - a lot of those also had different context, like "getting out of the saddle fast" etc.
If someone can prove me wrong, I'd love to see some evidence of it being correct and I will withdraw my "complaint."