How to change part of file Quick Look previews?
You're on the right path with qlmanage.  Unfortunately, using qlmanage is the last step in assigning a Quick Look generator to your file type.  You first have to obtain the generator, or write it from scratch.
So, to quickly answer both of your questions... yes, you can change what QL displays and yes, you can do it for a single file type.
To see what generators are associated with what file types, use the -m flag.  Here's a sample of the output
% qlmanage -m

  com.apple.iwork.pages.pages-tef -&gt; /System/Library/QuickLook/iWork.qlgenerator (209)
  com.microsoft.powerpoint.ppt -&gt; /System/Library/QuickLook/Office.qlgenerator (40)
  public.comma-separated-values-text -&gt; /System/Library/QuickLook/Office.qlgenerator (40)
  com.apple.ical.ics.todo -&gt; /System/Library/QuickLook/iCal.qlgenerator (906.3)
  com.adobe.encapsulated-postscript -&gt; /System/Library/QuickLook/EPS.qlgenerator (906.3)

Within each of those directories is code, very similar to a macOS Application,  that  generates a QL preview; so you can, in fact, create a preview to your specifications.
As for how to create that code, see the Introduction to Quick Look Programming Guide to get started.  The code is written in ANSI-C or Objective-C.