Hi Robin,
that’s actually quite easy:
Every live_loop will send its signal to a designated output channel. No. 3/4 from live_loop :arp e. g. go to the Tal Reverb loaded into Carla. Actually I forgot to remove the connections for channel 1/2, because Sonic Pi sends all its output automatically on channel 1/2. That is why during the performance some distortion can be heard: the live_loop :arp (channel 3/4) and all others could also be heard on the master channel 1/2 (without external reverb); that was a mistake which fortunately did not spoil the sound too much.
Does that help?
As to Carla there is a presentation from Filipe Coelho from last year’s Sonoj available (who attended also this year), which gives an exhaustive overview of this fine software.