I believe the current solution is already good enough if you do it right.
My advice for anyone who wants to hide the fee: Read the comment for the method xptClient_addDeveloperFeeEntry() in xptMiner. The function's output is static so you can pass your fee address once and write down the output. Then you hardcode the same output directly in the network functions and skip the call to xptClient_addDeveloperFeeEntry(). This will stop 99.9% of people that try to mess with the developer fee as they can't find the address in cleartext anymore. Additional measures can always be taken, like storing the address encrypted and only decrypting it on-demand.

Keep in mind that the developer fee was designed to work with any xpt pool without requiring the pool operator to interfere. At the moment only ypool supports it though.