JMO and I'm sure you've been in touch with TJ, but why not buy one unit of SoilFloc and apply it in small amounts over a length of time? Maybe apply a tenth of a unit on the pond side of the drain pipe where the leak is most likely and then repeat that process ten times. Maybe I'm picturing this all wrong but it seems like the source of your leak can only be in a small area. Worst case you're out a few hundred dollars.

Just a Pond Boss 'sponge'