Question Is it possible to
  1. Record a loop on the clean channel on my amp - then
  2. Play along with the lead channel (and gain)

That depends on where the Ditto Looper is connected. 
If you connect it in the loop of the amp (serial) , you can record with clean channel and play along with drive channel, since the Ditto Looper will be connected post pre-amp. if you connect the Ditto Looper between the guitar and the amp input, the Ditto recorded loops will follow the channel you set on the amp. If you record on the clean channel, and playback with a lead channel, the recorded loop will have the sound of the lead channel.