You should be able to mix flaps with throttle on that TX, so when you turn the flaps switch on you will get a certain % of flaps down depending on your throttle %.

Not sure if this will help with the P51 though, as I have no experience with warbirds (or flaps yet for that matter!).

I looked at the DX6i and the on/off flaps button was one reason I went for the DX7 instead, which has a 3 position switch and a trim button for flaps.