Re: PLIST that varies with options?

  Unless I am missing something, I wasn't able to find a way to specify
  PLIST entries that are subject to some options.

You are missing something :-)   reread the entire pkgsrc guide....  (But
there's a lot, so I don't blame you for missing it.)

  Is there any way or practice to deal with this situation?

See /usr/pkgsrc/doc/pkgsrc.xt:

  13.4. Variable substitution in PLIST