As the title says, I would like a command that allows a user to set the number of columns or rows in a toolbox GUI control.

With scripting commands we can only create the default "RscToolbox" that has 1 row and 2 columns, lbAdd and lbSetData/lbData work for adding text and data to the 2 existing toolbox buttons, however, using lbAdd more than twice doesn't create a new column.
SQF Example: https://sqfbin.com/wuyirepiyamodawipedo

i.e.
tbAddColumn & tbRemoveColumn OR tbSetColumns
tbAddRow & tbRemoveRow OR tbSetRows