I believe your worry is justified. Not that you needed it

Not having owned any before but have considered them in the past and actually just this week I am in a similar situation. Display has a few but overflow and sump the most. The way I've thought about it is turning off all the pumps, split the order, acclimate accordingly, and place half near the larger pest and the other half in the overflow.
As to where in the overflow I would reach in and juggle around a few pieces of rubble that I keep in there so they can start their journey there. My take is once they settle in they can withstand the overflow volume or so I hope. If I think they are moving around a bit too much then I'll just dial back my returns and let the display tank powerheads do the work.
Slow flow through sump is my general take anyway as it gives the skimmer more time to run through the water. Too fast and it is a lot of noise and skimmer is less efficient. At least in my opinion. My thought in this is that the berghia will work their way through the system to sump or back to display over the weir.
I do not know how the berghia find the pests. Is it like taking a walk or are they using smell or some other sense. I know not but my fear in my display is that I have pukani rock and it has a lot of nooks and crannies so they will be moving around for a long, long while.
Where did you find yours? Most of the places I looked are out. My display picked up again because I got rid of my matted filefish after he went through my frogspawn