Welcome to the GPF!
As far as your question - I have no clue, but my initial guess is you're taking a big risk. Number one, matrix blocks are reinforced internally. They are essentially three cubes joined with a shared top and bottom. You have no supporting pieces. And whether or not it would support the weight depends on one obvious question - how much weight?
Number two, matrix blocks are designed to allow maximum water flow while maintaining strength. Once you start drilling your container, it's getting weaker. And I'd be concerned that drilled holes would clog up very easily - just a piece of gravel in the wrong spot would stop water flow, not to mention any debris that might flow through.
I've seen people use milk crates in small pond less systems - I have no clue what the long term usefulness has been though.
Just my thoughts - we have a pondless waterfall and used a fair number of AquaBlox in our pond build as well, so I'm familiar with the construction concepts.