It can be done. I have done it. I haven't owned my Spektrum in a while though so I forget exactly how I did it. I'm almost embarrassed to say that since I normally tend to remember stuff.

I think though that I used mix #5 or #6 to slave flaps 100% to elevator (Or aileron, rudder, whatever you want as the master). I then went in through one of the menus and disabled the flap switch. It was something like that anyhow and it worked well.