Possible to use DSMX receiver with APM2?

I am building a DIY quadcopter (pics here) based around an Ardupilot Mega 2.8 flight controller. I have a Spektrum DXe 6-channel transmitter and an SPM4648 DSMX receiver. The receiver is binding successfully to the transmitter (orange LED glows solid) but no data is being read by the flight controller – as evidenced by Mission Planner radio calibration, which shows no activity on any of the transmitter sticks or switches. I connected the SPM4648 output to input 1 of the flight controller, but now I’m not sure there’s any reason to think that will work.

My hunch is that the the APM2 line of flight controllers (last compatible ardupilot firmware version 3.2.1) simply does not support DSMX. There was an attempt to make it work in 2013, but I don’t believe it was ever merged into the ardupilot mainline.

If that’s the case, it appears my options are:

  1. Get a new PPM-compatible receiver that works with the Spektrum DXe transmitter (suggestions?)
  2. Attempt to merge in the DSMX support into the ardupilot codebase and compile my own firmware (sounds scary!)
  3. Convert the SPM4648 DSMX output to PPM, but is that even a thing you can do?
  4. Something else I’m too noob-ish to think of.

I am new to all of this, so please bear with me if I’ve said anything silly. All tips, info and advice much appreciated!

I went with option 2 – replaced the SPM4648 DSMX receiver with a PPM-capable AR7700 and now the controller and the receiver are talking, huzzah!