This is a very handy tip.
You can add delay to a bus or Main LR mix on an XAir mixer by inserting a Pitch Shifter FX processor. This is the best processor to use for this because the Delay control has finer resolution than the normal delays. If you set Semi and Cent to zero, Mix to 100% and Hi Cut to 20kHz, the processor will just act as a delay. Use half of a Dual processor for a bus or the Stereo version for Main LR or a linked bus pair.