I figured out my problem! I was close, but when you "grow" a structure array in a loop (e.g. all_params(i) = this_param;) it populates it in the column direction. So, the example system works by either:
Specifying the "partition dimension" in the "for each" block to "2"
OR
Specifiying the structure array to be a row vector (e.g. all_params(i,1) = this_param;)