Have you been in contact with the pond owner in the watershed above your pond? that is what I did, went and talked to the owner about what he had stocked in his pond because inevitably, if not this year, next year, a rain event will bring yoy into your pond, in my case I was in luck, he had stocked pretty similar to what I was shooting for and I have seen LMB in my pond which I haven't stocked yet with predator fish.
I haven't seen any BC yet in my pond but that doesnt mean there are not a few of them there, or many.
I assumed it might make a difference what time of year that the rain event happened as to what young fry washes out the overflow, I know last year after I had just built the pond and it was filling up we had tons of rain events, all the way from March thru the end of May, I think I may have received a ton of stocking fry from his pond which was obviously full when the rains started.

All the really good ideas I've ever had came to me while I was milking a cow.