AESA radars send radar impulses on different frequencies that appear to the observer to be random (and thus hard to figure out and jam).
They could use something similar for communication. Send the first byte on a frequency, the second byte on another one, etc.... The frequencies could be calculated to be un-distinguishable from the background radio radiation. Of course, both the sender and the receiver have to agree on this mechanism.
The really clever counterpart is how to transmit through your own spread-spectrum jamming. Quiet slots in the jamming must be manipulated in the same way as the active broadcast. For example, this allows communication to a ground convoy that is jamming all frequencies to prevent wireless remote-control IED detonation.
They could use something similar for communication. Send the first byte on a frequency, the second byte on another one, etc.... The frequencies could be calculated to be un-distinguishable from the background radio radiation. Of course, both the sender and the receiver have to agree on this mechanism.