As for the QT anyone who builds the watchonly branch and manually adds watchonly addresses is indeed at a big risk since those accounts aren't meant to be spent. However the change you propose is not good because I use that branch for Halo. On the other hand users should build from the branch without watchonly.
If your DEX requires the QT wallet runs watchonly then perhaps a change needs to be made to the QT GUI to accommodate it.
barterDEX does create watchonly addresses and since any user could manually add a watchonly address with a single command, it seems a very good idea to update the QT wallet to at least denote which addresses are watch only.
Oh, I think BLK was missing gettxout rpc call last time we checked. Any chance of that being added (if it isnt already there)?