I think progress is being made. I have created a BGHQ with sub-formations in MobHack.
Please note I modified the Purchase Screen to Default, and the Formation Type was changed to Platoon. A little hack, place the cursor inside the Platoon type window and enter 1 directly.
Now the Editor will by default load the modified BGHQ at the Purchase screen with verification in the Roster.
Further, a peek at the scenario in ScnHack reveals a BGHQ with sub-formations.
Now, I need to purchase the remaining force.
What I did was to save a copy of the OOB in my Customs folder. Once, I completed the BGHQ purchase, I rebuilt the Headquarters Formation to the game default specifications.
This might work.
Thanks to all posters for their time, effort, and insight.