This is fairly close to where we are heading (if you look at my recent post, not sure who posted first
)
There are two options:
1) Encode the feed into the chain and use a median
2) Have a soft feed where delegates simply ignore transactions outside of range.
The first option is probably the most robust against an attack by a single delegate. The second is possible today without any hard fork.
I think I like your proposal Agent86... a 30% range of allowable orders works well and moves all attack vectors "off-chain", ie: if you can pump and dump BTSX causing a 50% fall in value. Nothing we do on chain can prevent that except increasing margin requirements beyond what a pump & dump can do.