i had an old 1700mah life battery lying around so i thoght i would try it on my dx4r pro, however the battery was just slightly too tall to work.
but the removal of the battery tray is easy, as previously mentioned, get the 3 screws out at the base part of the radio and one further up above the grip. you can take out even more above that which will give you a little more flex between the 2 halves.
one word of warning, watch the switch and the little tear drop insert below it. mine went flying out and had a job finding them again.
also the negative wire soldered to the AA battery tray is insanely short, so much so i dunno how they got the tray in there. i soldered in an extension to this wire so it was easier to but back in the radio tray until a source a slightly skinner life battery.