I went back to this since I was just a wee lad when I did this, and it's not quite what I remembered but I was close. You have to change the string "SECRET" in the header to anything else. At this point you can load the file into the board editor and see how things were done.

Here is an image of TOWN.ZZT loaded into the editor: https://i.imgur.com/mb9OFno.png

And here is the pascal string "SECRET" changed to "NECRET": https://i.imgur.com/kbh5xAm.png

I'm surprised that 12-year old me thought to do this.