Wiktionary
back-channel
vb. (alternative spelling of backchannel English)
WordNet
back-channel
adj. via a back channel; "thefailure of back-channel negotiations"
Usage examples of "back-channel".
Also, as Khaki was an Arab from one of those sheikdoms that Washington was always trying to reach on the sly, he had certain built-in protections that came when the government concluded back-channel negotiations with politically unpopular people.
Cailetet and other BMs have indicated through back-channels that they will not support your proposal.