[Therion] Hiding centreline within a scrap, while showing it for surveys

[Tarquin Wilton-Jones]

Hi folks,

I know this must be a common desire, but I can't seem to find it on the
wiki or Therion book.

map foo
  bar at baz #a map containing scraps
  qux     #a survey
endmap

This will show the scraps within bar at baz, and the centreline for qux.
All good.

I want to hide the centreline within bar at baz, since it has passage
walls, and no need to show a centreline. But I need to continue showing
the centreline for qux, since it has no passage wall data, only the
centreline.

I am sure I have seen this on John's surveys, but I can't seem to work
out how to do it (all I can see is a mail from Stacho in 2007 asking for
clarification whether someone actually wanted that functionality).

Is there a way?