I'm not sure what you're trying to say. It's a given that BTS cannot be transferred in the OP scenario. But user will still be able to transfer BTC to Poloniex and trade a POLO.BTS token. They'll also still be able to move funds from Poloniex to the DEX by buying BitUSD (or BitCNY) and then transfering THAT to the DEX. Sure, it would be less practical for users, but that just means fewer people moving funds to the DEX. And from Poloniex's standpoint there would be NO CHANGE, other than disabling BTS transfers on their withdrawal page.
Where does polonix get the BTS to back the POLO.BTS token?
Either:
1. They have to hold a massive stash of real BTS to cope with wildly varying demand
2. They have to somehow convert real BTC into a DEX BTC.IOU (no existing infrastructure), then trade it for BTS on the DEX
Neither of these options is going to be tenable for them; they have to become a bridge, and what you are suggesting is that polonix print BTS out of nothing.
edit: think about it some more; how can their POLO.BTS token affect the price of BTS when no arbitrage is possible? It would be totally pointless.